feat: Implement WebFinger discovery and ActivityPub user actor endpoint

- Added a new router for handling well-known endpoints, specifically WebFinger.
- Implemented the `webfinger` function to respond to WebFinger queries.
- Created a new route for WebFinger in the router.
- Refactored user retrieval logic to support both user ID and username in a single endpoint.
- Updated user router to use the new `get_user_by_param` function.
- Added tests for WebFinger discovery and ActivityPub user actor endpoint.
- Updated dependencies in Cargo.toml files to include necessary libraries.

thoughts-backend/tests/api/mod.rs

@@ -1,3 +1,4 @@
+mod activitypub;
 mod auth;
 mod feed;
 mod follow;