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

conf/service_conf.yaml

@@ -73,6 +73,13 @@ redis:
 #     app_access_token_url: https://open.feishu.cn/open-apis/auth/v3/app_access_token/internal
 #     user_access_token_url: https://open.feishu.cn/open-apis/authen/v1/oidc/access_token
 #     grant_type: 'authorization_code'
+#   custom_channel:
+#     type: oidc
+#     issuer: https://provider.com/v1/oidc
+#     client_id: xxxxxxxxxxxxxxxxxxxxxxxxx
+#     client_secret: xxxxxxxxxxxxxxxxxxxxxxxx
+#     scope: "openid email profile"
+#     redirect_uri: https://your-app.com/oauth/callback/custom_channel
 # authentication:
 #   client:
 #     switch: false