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/src/assistant_tools.rs

@@ -0,0 +1,13 @@
+mod now_tool;
+
+use assistant_tool::ToolRegistry;
+use gpui::AppContext;
+
+use crate::now_tool::NowTool;
+
+pub fn init(cx: &mut AppContext) {
+    assistant_tool::init(cx);
+
+    let registry = ToolRegistry::global(cx);
+    registry.register_tool(NowTool);
+}