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/doc/src/user.rs

@@ -12,7 +12,7 @@ use models::schemas::{
 #[openapi(
     paths(
         users_get,
-        users_id_get,
+        get_user_by_param,
         user_thoughts_get,
         user_follow_post,
         user_follow_delete