feat(auth): implement OAuth 2.0 authorization code flow and dynamic client registration

- Add OAuth 2.0 support with authorization code flow and dynamic client registration.
- Introduce new handlers for OAuth metadata, client registration, authorization, and token issuance.
- Enhance authentication middleware to support OAuth client credentials.
- Create in-memory stores for authorization codes and tokens.
- Update configuration to include OAuth client details.
- Ensure validation checks for OAuth clients in the configuration.

configs/config.example.yaml

@@ -14,7 +14,6 @@ mcp:
   transport: "streamable_http"
 
 auth:
-  mode: "api_keys"
   header_name: "x-brain-key"
   query_param: "key"
   allow_query_param: false
@@ -22,6 +21,12 @@ auth:
     - id: "local-client"
       value: "replace-me"
       description: "main local client key"
+  oauth:
+    clients:
+      - id: "oauth-client"
+        client_id: ""
+        client_secret: ""
+        description: "used when auth.mode=oauth_client_credentials"
 
 database:
   url: "postgres://postgres:postgres@localhost:5432/amcs?sslmode=disable"