docs: Update docs for default pairs (#196)

* docs: Update docs for default pairs

* update defaultable pair

* rollback changes of specs

docs/go-storage/internal/specs.md

@@ -186,7 +186,7 @@ The name of current service, should be unique.
 implement = ["multiparter"]
 ```
 
-We should also add related `UnimplementedMultiparter` stub into our `Storage` struct.
+- We should also add related `UnimplementedMultiparter` stub into our `Storage` struct.
 
 `namespace` MUST add `new` section to describe how to initiate this namespace:
 

docs/go-storage/operations/index.md

@@ -25,25 +25,29 @@ We have connection string support.
 
 The format of the connection string is (optional parts marked by squared brackets):
 
-`<type>://[<name>][<work_dir>][?key1=value1&...&keyN=valueN]`
+`<type>://[<name>][<work_dir>][?enable_feature1&...&enable_featureN&key1=value1&...&keyN=valueN]`
 
 - name: storage name, e.g., bucket name. MUST NOT contain /
 - work_dir: For object storage, it is prefix; for fs, it is directory path. MUST start with / for every storage services.
+- For the `enable_feature` query string:
+  - `feature` is the feature name defined in `toml`, and the format SHOULD be exactly the same.
 - For the `key=value` pairs:
   - If `=value` is missing, we just ignore the pair. But `key=` means a pair with a blank value.
   - The `key` is the pair name defined in `toml` and the format SHOULD be exactly the same.
-- If their are multiple pairs with the same key, the first one will be picked.
+  - Or the `key` is the default pair name prefixed with `default_` and followed by pair name described above.
+- If there are multiple pairs with the same key, the first one will be picked.
 
 So a valid connection string could be:
 
 - `s3://bucket_name`
 - `s3://bucket_name/prefix`
 - `s3://?credential=hmac:xxxx:xxxx&endpoint=http:s3.us-east-2.amazonaws.com`
 - `s3://bucket_name/prefix?credential=hmac:xxxx:xxxx&endpoint=http:s3.us-east-2.amazonaws.com`
+- `s3://bucket_name/prefix?enable_virtual_dir&credential=hmac:xxxx:xxxx&endpoint=http:s3.us-east-2.amazonaws.com&default_storage_class=STANDARD`
 - `fs://`
 - `fs:///tmp`
 
-For more details, take a look at [GSP-90](https://github.com/beyondstorage/specs/blob/master/rfcs/90-re-support-initialization-via-connection-string.md).
+For more details, take a look at [GSP-90](https://github.com/beyondstorage/specs/blob/master/rfcs/90-re-support-initialization-via-connection-string.md) and [GSP-700](https://github.com/beyondstorage/go-storage/blob/master/docs/rfcs/700-config-features-and-defaultpairs-via-connection-string.md).
 
 :::caution
 

docs/go-storage/pairs/index.md

@@ -35,7 +35,7 @@ We will document all global pair here and leave system pairs in service's docume
 
 `go-storage` provides a mechanism to allow user pass default pairs for every operation during `NewServicer` and `NewStorager`.
 
-Any service that supports this mechanism will generate system pairs called  `DefaultServicePairs` and `DefaultStoragePairs`:
+Any service that supports this mechanism will generate system pairs called `DefaultServicePairs` and `DefaultStoragePairs`:
 
 ```go
 type DefaultStoragePairs struct {
@@ -72,4 +72,90 @@ store, err := s3.NewStorager(
 )
 ```
 
-As in example, every call to `Write` will specify the `storage_class` to `STANDARD_IA`.
+As in example, every call to `Write` will specify the `storage_class` to `STANDARD_IA`.
+
+### Defaultable Pair
+
+`defaultable` property for pair is introduced to set default value for pair, and the pair of operations with the same name will share the default value.
+
+Default pairs will be generated for `defaultable` pairs, and the generated pair for defaultable global pair is still global:
+
+```go
+// Defaultable global pair in pairs.
+func WithDefualtContentType(v string) Pair {
+    return Pair{
+        Key:   "enable_content_type",
+        Value: v,
+    }
+}
+
+// Defualtable system pair in service.
+func WithDefaultStorageClass(v string) Pair {
+	return Pair{
+        Key:   "default_storage_class",
+        Value: v,
+	}
+}
+```
+
+User can use default pairs like:
+
+```go
+store, err := s3.NewStorager(
+    s3.WithDefaultStorageClass("STANDARD_IA"),
+    pairs.WithDefualtContentType("application/octet-stream"),
+)
+```
+
+As in example, pairs of operations in the service with the same name `storage_class` or `content_type` will share the default values.
+
+## Feature Pairs
+
+`go-storage` provides a mechanism of userland optional abilities for service during `NewServicer` and `NewStorager`.
+
+Any service that supports this mechanism will generate system pairs called `StorageFeatures` and `ServiceFeatures`:
+
+```go
+type StorageFeatures struct {
+	LoosePair bool
+	VirtualDir bool
+}
+
+func WithStorageFeatures(v StorageFeatures) Pair {
+    return Pair{
+        Key:   "storage_features",
+        Value: v,
+    }
+}
+```
+
+Enable feature pairs will be generated for each feature in service:
+
+```go
+func WithEnableVirtualDir() Pair {
+    return Pair{
+        Key:   "enable_virtual_dir",
+        Value: true,
+    }
+}
+```
+
+User can enable features like this:
+
+```go
+store, err := s3.NewStorager(
+    s3.WithStorageFeatures(s3.StorageFeatures{
+        VirtualDir:  true,
+    }),
+)
+```
+
+or:
+
+```go
+store, err := s3.NewStorager(
+	s3.WithEnableVirtualDir(),
+)
+```
+
+As in the above examples, the service will support simulated dir behavior.