### 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)

### What problem does this PR solve?

Add `/login/channels` route and improve auth logic to support frontend
integration with third-party login providers:

- Add `/login/channels` route to provide authentication channel list
with `display_name` and `icon`
- Optimize user info parsing logic by prioritizing `avatar_url` and
falling back to `picture`
- Simplify OIDC token validation by removing unnecessary `kid` checks
- Ensure `client_id` is safely cast to string during `audience`
validation
- Fix typo

---
- Related pull request: #7379 

### Type of change

- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update

### What problem does this PR solve?

Add support for OAuth2 and OpenID Connect (OIDC) authentication,
allowing OAuth/OIDC authentication using the specified routes:
- `/login/<channel>`: Initiates the OAuth flow for the specified channel
- `/oauth/callback/<channel>`: Handles the OAuth callback after
successful authentication

The callback URL should be configured in your OAuth provider as:
```
https://your-app.com/oauth/callback/<channel>
```

For detailed instructions on configuring **service_conf.yaml.template**,
see: `./api/apps/auth/README.md#usage`.

- Related issues
#3495  

### Type of change

- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update