Factor tool definitions out of assistant (#21189)

This PR factors the tool definitions out of the `assistant` crate so
that they can be shared between `assistant` and `assistant2`.

`ToolWorkingSet` now lives in `assistant_tool`. The tool definitions
themselves live in `assistant_tools`, with the exception of the
`ContextServerTool`, which has been moved to the `context_server` crate.

As part of this refactoring I needed to extract the
`ContextServerSettings` to a separate `context_server_settings` crate so
that the `extension_host`—which is referenced by the `remote_server`—can
name the `ContextServerSettings` type without pulling in some undesired
dependencies.

Release Notes:

- N/A

crates/zed/Cargo.toml

@@ -20,6 +20,7 @@ anyhow.workspace = true
 assets.workspace = true
 assistant.workspace = true
 assistant2.workspace = true
+assistant_tools.workspace = true
 async-watch.workspace = true
 audio.workspace = true
 auto_update.workspace = true