rfc: add built-in "protobuf" data type support

docs/RFCS/20240720_protobuf_type.md

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+- Feature Name: Built-in Protobuf Type
+- Status: Draft
+- Start Date: 2024-07-20
+- Authors: Xiaochen Cui
+- RFC PR: N/A
+- Cockroach Issue: [#47534]https://github.com/cockroachdb/cockroach/issues/47534
+
+Table of Contents
+
+- [Summary](#summary)
+- [Motivation](#motivation)
+- [Guide-level Explanation](#guide-level-explanation)
+- [Reference-level Explanation](#reference-level-explanation)
+- [Drawbacks](#drawbacks)
+- [Rationale and Alternatives](#rationale-and-alternatives)
+- [Work Itmes](#work-itmes)
+- [Resources](#resources)
+
+# Summary
+
+With the built-in "protobuf" type, the protobuf data which is stored in the database can be rendered as a human-readable format directly. It can also be queried and indexed directly.
+
+# Motivation
+
+Currently, we have some interal columns that store protobuf data as bytes, the accessing of the data is through built-in functions `crdb_internal.pb_to_json`, `crdb_internal.json_to_pb`, etc. The built-in protobuf type can provide a more convenient way to access the data.
+
+# Guide-level Explanation
+
+## Column Definition
+
+The `protobuf` type should be initialized with a file descriptor and a message type name.
+
+```sql
+CREATE TABLE t (
+    id INT PRIMARY KEY,
+    data PROTOBUF(file_descriptor='file_descriptor_content', message_type='message_type_name')
+);
+```
+
+## Insert Data
+
+The data can be inserted into the `protobuf` type column with the `protobuf` format. An error will be thrown if the data format is not correct.
+
+```sql
+INSERT INTO t VALUES (1, 'protobuf_data');
+```
+
+## Query Data
+
+General query:
+
+```sql
+SELECT * FROM t;
+----
++----+----------------+
+| id | data           |
++----+----------------+
+| 1  | protobuf_data  |
++----+----------------+
+```
+
+## Alter Column Type
+
+The `protobuf` type can be altered to `bytes` type without any parameter. We can also alter the `bytes` type to `protobuf` type with the file descriptor and message type name.
+
+```sql
+ALTER TABLE t ALTER COLUMN data TYPE BYTES;
+ALTER TABLE t ALTER COLUMN data TYPE PROTOBUF(file_descriptor='file_descriptor_content', message_type='message_type_name');
+```
+
+## Alter Protobuf Definition
+
+The `protobuf` type can be altered to change the file descriptor and message type name, only when the updated type
+definition is compatible with the original one.
+
+```sql
+ALTER TABLE t ALTER COLUMN data TYPE PROTOBUF(file_descriptor='new_file_descriptor_content', message_type='new_message_type_name');
+```
+
+# Reference-level Explanation
+
+# Drawbacks
+
+# Rationale and Alternatives
+
+# Work Itmes
+
+- [ ] Implement the built-in `protobuf` type.
+- [ ] Implement the `PROTOBUF` type definition in the parser.
+- [ ] Privode the tool to convert existing `bytes` type to `protobuf` type.
+
+# Resources
+
+- [cpustejovsky/filedescriptorjson: Wrapper Code to Convert a protobuf binary to a JSON message based on the FileDescritpro](https://github.com/cpustejovsky/filedescriptorjson)