Add SingleStore connector (#257)

This PR adds singlestore connector.

---------

Co-authored-by: Adalbert Makarovych <amakarovych0ua@singlestore.com>

registry/hasura/singlestore/README.md

@@ -0,0 +1,147 @@
+# SingleStore Data Connector
+
+<a href="https://www.singlestore.com/"><img src="https://github.com/singlestore-labs/singlestore-hasura-connector/blob/main/docs/singlestore_logo_horizontal_color_on-white_rgb.png" align="right" width="200"></a>
+
+<!-- TODO: update when connector will be published -->
+[![Docs](https://img.shields.io/badge/docs-v3.x-brightgreen.svg?style=flat)](https://hasura.io/docs/3.0/latest/connectors/singesltore/)
+[![ndc-hub](https://img.shields.io/badge/ndc--hub-singlestore-blue.svg?style=flat)](https://hasura.io/connectors/singlestore)
+[![License](https://img.shields.io/badge/license-Apache--2.0-purple.svg?style=flat)](LICENSE)
+[![Status](https://img.shields.io/badge/status-alpha-yellow.svg?style=flat)](./readme.md)
+
+The Hasura SingleStore Connector ("the connector") enables you to connect to a SingleStore database and gives instant access to a GraphQL API on top of your data.
+
+This connector is built using the [Typescript Data Connector SDK](https://github.com/hasura/ndc-sdk-typescript) and, it implements the [Data Connector Spec](https://github.com/hasura/ndc-spec).
+
+<!-- TODO: update when connector will be published -->
+- [See the listing in the Hasura Hub](https://hasura.io/connectors/singlestore)
+- [Hasura V3 Documentation](https://hasura.io/docs/3.0)
+
+## Features
+
+The following matrix lists the features supported by the Hasura SingleStore connector:
+
+| Feature                         | Supported | Notes |
+| ------------------------------- | --------- | ----- |
+| Native Queries + Logical Models | ❌     |       |
+| Simple Object Query             | ✅     |       |
+| Filter / Search                 | ✅     |       |
+| Simple Aggregation              | ✅     |       |
+| Sort                            | ✅     |       |
+| Paginate                        | ✅     |       |
+| Table Relationships             | ✅     |       |
+| Views                           | ✅     |       |
+| Distinct                        | ✅     |       |
+| Remote Relationships            | ✅     |       |
+| Mutations                       | ❌     | coming soon       |
+
+## Prerequisites
+
+Ensure that the following prerequisites are met before using the connector:
+
+1. Install [DDN CLI](https://hasura.io/docs/3.0/cli/installation) and [Docker](https://docs.docker.com/engine/install/).
+2. Initialize a [supergraph](https://hasura.io/docs/3.0/getting-started/init-supergraph).
+3. Initialize a [subgraph](https://hasura.io/docs/3.0/getting-started/init-subgraph).
+4. An active [SingleStore](https://www.singlestore.com/) deployment that serves as the data source for the API.
+
+
+## Using the SingleStore connector
+
+The following steps explain how to initialize and configure the connector for local development. For information on deploying a connector after it has been configured, refer to [Deploy a Connector](https://hasura.io/docs/3.0/getting-started/deployment/deploy-a-connector).
+<!-- TODO: test steps after the connector will be published to hub -->
+### Step 1: Authenticate your CLI session
+
+```bash
+ddn auth login
+```
+
+### Step 2: Configure the connector
+
+After the supergraph and subgraph have been initialized (refer to [Prerequisites](#Prerequisites), run the initialization command in interactive mode. Specify a name for the connector in the command.
+
+```bash
+ddn connector init <connector-name>  -i
+```
+
+#### Step 2.1: Select the SingleStore connector
+
+From the list of connectors, select `singlestore`.
+
+#### Step 2.2: Select a port for the connector
+The CLI prompts you to specify a port for running the connector. Either select the default port suggested or choose a port that is not currently in use.
+
+#### Step 2.3: Provide the environment variable(s) for the connector
+
+Specify the connection string used to connect to the SingleStore database using the `SINGLESTORE_URL` variable. The connection string must be in the `mysql://[<username>[:<password>]][@<host>:[<port>]]/[<database>][?<key1>=<value1>[&<key2>=<value2>]]` format. 
+
+For example:
+
+```env
+SINGLESTORE_URL=singlestore://username:password@hostname/database
+```
+The connector uses [MySQL2](https://sidorares.github.io/node-mysql2/docs) library to establish a connection.
+For more information, refer to [Connection options](https://www.npmjs.com/package/mysql#connection-options) and [Pool options](https://www.npmjs.com/package/mysql#pool-options).
+
+Alternatively, you can also set following environment variables instead of the `SINGLELSTORE_URL` variable:
+
+| Name                                | Default     | Description |
+|-------------------------------------|-------------|-------------|
+| SINGLESTORE_HOST                    | localhost   | Hostname of the SingleStore database to connect with. |
+| SINGLESTORE_PORT                    | 3306        | Port number of the SingleStore database. |
+| SINGLESTORE_PASSWORD                |             | Password of the SingleStore database user. |
+| SINGLESTORE_DATABASE                |             | Name of the SingleStore database to connect with. |
+| SINGLESTORE_SSL_CA                  |             | Path to the trusted CA certificate file. |
+| SINGLESTORE_SSL_CERT                |             | Path to the certificate chain file in PEM format. |
+| SINGLESTORE_SSL_KEY                 |             | Path to the private key file in PEM format. |
+| SINGLESTORE_SSL_CIPHERS             |             | Cipher suite specification. If specified, it replaces the default value. |
+| SINGLESTORE_SSL_PASSPHRASE          |             | Cipher suite specification. If specified, it replaces the default value. |
+| SINGLESTORE_SSL_REJECT_UNAUTHORIZED | true        | If enabled, the server rejects any connection that is not authorized with the list of supplied CAs. |
+
+If `SINGLELSTORE_URL` is set these variables are ignored.
+
+### Step 3: Introspect your data source
+
+After configuring the connector, use the CLI to introspect the SingleStore database and create a source-specific configuration file for the connector (`configuration.json`).
+
+```bash
+ddn connector introspect <connector-name>
+```
+
+## Step 4: Add your resources
+
+This command will create `.hml` files for each table and view in your database 
+and an `.hml` file with information about SingleStore data types.
+
+```bash
+ddn connector-link add-resources <connector-name>
+```
+
+## Step 5. Add relationships
+
+SingleStore does not support foreign keys. Relationships between tables must be added manually. You can define relationships by appending relationship information to the `.hml` files generated in the previous step.
+For information on defining relationships, refer to [Relationships](https://hasura.io/docs/3.0/supergraph-modeling/relationships/).
+For example, to add a relationship from a `message` table to the `user` table, append following text to the 
+`DbMessage.hml` file:
+```hml
+---
+kind: Relationship
+version: v1
+definition:
+  name: user
+  sourceType: DbMessage
+  target:
+    model:
+      name: DbUser
+      subgraph: app
+      relationshipType: Object
+  mapping:
+    - source:
+        fieldPath:
+          - fieldName: userId
+      target:
+        modelField:
+          - fieldName: id
+  description: The user details for a message
+```
+## License
+
+The SingleStore connector is available under the [Apache License 2.0](https://www.apache.org/licenses/LICENSE-2.0).

registry/hasura/singlestore/metadata.json

@@ -0,0 +1,43 @@
+{
+    "overview": {
+        "namespace": "hasura",
+        "description": "The Hasura SingleStore Connector enables you to connect to a SingleStore database and gives instant access to a GraphQL API on top of your data.",
+        "title": "SingleStore Data Connector",
+        "logo": "logo.png",
+        "tags": [
+            "database"
+        ],
+        "latest_version": "v0.0.1"
+    },
+    "author": {
+        "support_email": "integrations@singlestore.com",
+        "homepage": "https://www.singlestore.com/",
+        "name": "SingleStore"
+    },
+    "is_verified": false,
+    "is_hosted_by_hasura": false,
+    "packages": [
+        {
+            "version": "0.0.1",
+            "uri": "https://github.com/singlestore-labs/singlestore-hasura-connector/releases/download/v0.0.1/connector-definition.tgz",
+            "checksum": {
+                "type": "sha256",
+                "value": "7cc9f73cb966c2b69ef54d5f0e346407d3555ed1ac08f361f16157540bb80041"
+            },
+            "source": {
+                "hash": "a9caa889c13a251040003c0934c7e67fccfd7de4"
+            }
+        }
+    ],
+    "source_code": {
+        "is_open_source": true,
+        "repository": "https://github.com/singlestore-labs/singlestore-hasura-connector/",
+        "version": [
+            {
+                "tag": "v0.0.1",
+                "hash": "a9caa889c13a251040003c0934c7e67fccfd7de4",
+                "is_verified": false
+            }
+        ]
+    }
+}

registry/hasura/singlestore/releases/v0.0.1/connector-packaging.json

@@ -0,0 +1,11 @@
+{
+    "version": "0.0.1",
+    "uri": "https://github.com/singlestore-labs/singlestore-hasura-connector/releases/download/v0.0.1/connector-definition.tgz",
+    "checksum": {
+        "type": "sha256",
+        "value": "7cc9f73cb966c2b69ef54d5f0e346407d3555ed1ac08f361f16157540bb80041"
+    },
+    "source": {
+        "hash": "a9caa889c13a251040003c0934c7e67fccfd7de4"
+    }
+}