Add an extensions API to the collaboration server (#7807)

This PR adds a REST API to the collab server for searching and
downloading extensions. Previously, we had implemented this API in
zed.dev directly, but this implementation is better, because we use the
collab database to store the download counts for extensions.

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>

crates/collab/src/db/tables.rs

@@ -10,6 +10,8 @@ pub mod channel_message;
 pub mod channel_message_mention;
 pub mod contact;
 pub mod contributor;
+pub mod extension;
+pub mod extension_version;
 pub mod feature_flag;
 pub mod follower;
 pub mod language_server;