Feat: Add support for OAuth2 and OpenID Connect (OIDC) authentication (#7379)

### 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

api/settings.py

@@ -56,7 +56,7 @@ CLIENT_AUTHENTICATION = None
 HTTP_APP_KEY = None
 GITHUB_OAUTH = None
 FEISHU_OAUTH = None
-
+OAUTH_CONFIG = None
 DOC_ENGINE = None
 docStoreConn = None
 
@@ -119,7 +119,7 @@ def init_settings():
         RAG_FLOW_SERVICE_NAME,
         {}).get("secret_key", str(date.today()))
 
-    global AUTHENTICATION_CONF, CLIENT_AUTHENTICATION, HTTP_APP_KEY, GITHUB_OAUTH, FEISHU_OAUTH
+    global AUTHENTICATION_CONF, CLIENT_AUTHENTICATION, HTTP_APP_KEY, GITHUB_OAUTH, FEISHU_OAUTH, OAUTH_CONFIG
     # authentication
     AUTHENTICATION_CONF = get_base_config("authentication", {})
 
@@ -131,6 +131,8 @@ def init_settings():
     GITHUB_OAUTH = get_base_config("oauth", {}).get("github")
     FEISHU_OAUTH = get_base_config("oauth", {}).get("feishu")
 
+    OAUTH_CONFIG = get_base_config("oauth", {})
+
     global DOC_ENGINE, docStoreConn, retrievaler, kg_retrievaler
     DOC_ENGINE = os.environ.get('DOC_ENGINE', "elasticsearch")
     # DOC_ENGINE = os.environ.get('DOC_ENGINE', "opensearch")