Add REST APIs for getting and adding contributors

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/collab/migrations/20240122174606_add_contributors.sql

@@ -0,0 +1,5 @@
+CREATE TABLE contributors (
+    user_id INTEGER REFERENCES users(id),
+    signed_at TIMESTAMP NOT NULL DEFAULT NOW(),
+    PRIMARY KEY (user_id)
+);