Lay the groundwork for collaborating on assistant panel (#13991)

This pull request introduces collaboration for the assistant panel by
turning `Context` into a CRDT. `ContextStore` is responsible for sending
and applying operations, as well as synchronizing missed changes while
the connection was lost.

Contexts are shared on a per-project basis, and only the host can share
them for now. Shared contexts can be accessed via the `History` tab in
the assistant panel.

<img width="1819" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/c7ae46d2-cde3-4b03-b74a-6e9b1555c154">


Please note that this doesn't implement following yet, which is
scheduled for a subsequent pull request.

Release Notes:

- N/A

crates/collab/Cargo.toml

@@ -71,6 +71,7 @@ util.workspace = true
 uuid.workspace = true
 
 [dev-dependencies]
+assistant = { workspace = true, features = ["test-support"] }
 async-trait.workspace = true
 audio.workspace = true
 call = { workspace = true, features = ["test-support"] }