Refa: Deprecate /github_callback in favor of /oauth/callback/<channel> for GitHub OAuth integration (#7587)

### What problem does this PR solve?

Deprecate `/github_callback` route in favor of
`/oauth/callback/<channel>` for GitHub OAuth integration:

- Added GitHub OAuth support in the authentication module
- Introduced `GithubOAuthClient` with methods to fetch and normalize
user info
  - Updated `CLIENT_TYPES` to include GitHub OAuth client
- Deprecated `/github_callback` route and suggested using the generic
`/oauth/callback/<channel>` route

---
- Related pull requests: 
  - #7379
  - #7553 

### Usage

- [Create a GitHub OAuth
App](https://github.com/settings/applications/new) to obtain the
`client_id` and `client_secret`, configure the authorization callback
url: `https://your-app.com/v1/user/oauth/callback/github`
- Edit `service_conf.yaml.template`:
  ```yaml
  # ...
  oauth:
    github:
      type: "github"
      icon: "github"
      display_name: "Github"
      client_id: "your_client_id"
      client_secret: "your_client_secret"
      redirect_uri: "https://your-app.com/v1/user/oauth/callback/github"
  # ...
  ```

### Type of change

- [x] Documentation Update
- [x] Refactoring (non-breaking change)

api/apps/auth/README.md

@@ -32,8 +32,16 @@ oidc_config = {
     "redirect_uri": "https://your-app.com/v1/user/oauth/callback/<channel>"
 }
 
+# Github OAuth configuration
+github_config = {
+    "type": "github"
+    "client_id": "your_client_id",
+    "client_secret": "your_client_secret",
+    "redirect_uri": "https://your-app.com/v1/user/oauth/callback/<channel>"
+}
+
 # Get client instance
-client = get_auth_client(oauth_config)  # or oidc_config
+client = get_auth_client(oauth_config)
 ```
 
 ### Authentication Flow