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/assistant_tools/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "assistant_tools"
+version = "0.1.0"
+edition = "2021"
+publish = false
+license = "GPL-3.0-or-later"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/assistant_tools.rs"
+
+[dependencies]
+anyhow.workspace = true
+assistant_tool.workspace = true
+chrono.workspace = true
+gpui.workspace = true
+schemars.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+workspace.workspace = true