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/apps/auth/__init__.py

@@ -0,0 +1,38 @@
+#
+#  Copyright 2025 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+
+from .oauth import OAuthClient
+from .oidc import OIDCClient
+
+
+CLIENT_TYPES = {
+    "oauth2": OAuthClient,
+    "oidc": OIDCClient
+}
+
+
+def get_auth_client(config)->OAuthClient:
+    channel_type = str(config.get("type", "")).lower()
+    if channel_type == "":
+        if config.get("issuer"):
+            channel_type = "oidc"
+        else:
+            channel_type = "oauth2"
+    client_class = CLIENT_TYPES.get(channel_type)
+    if not client_class:
+        raise ValueError(f"Unsupported type: {channel_type}")
+
+    return client_class(config)