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/assistant/src/completion_provider.rs

@@ -1,13 +1,13 @@
 mod anthropic;
 mod cloud;
-#[cfg(test)]
+#[cfg(any(test, feature = "test-support"))]
 mod fake;
 mod ollama;
 mod open_ai;
 
 pub use anthropic::*;
 pub use cloud::*;
-#[cfg(test)]
+#[cfg(any(test, feature = "test-support"))]
 pub use fake::*;
 pub use ollama::*;
 pub use open_ai::*;