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

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