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/src/context.rs

@@ -2,7 +2,6 @@
 mod context_tests;
 
 use crate::slash_command_working_set::SlashCommandWorkingSet;
-use crate::ToolWorkingSet;
 use crate::{
     prompts::PromptBuilder,
     slash_command::{file_command::FileCommandMetadata, SlashCommandLine},
@@ -12,6 +11,7 @@ use anyhow::{anyhow, Context as _, Result};
 use assistant_slash_command::{
     SlashCommandContent, SlashCommandEvent, SlashCommandOutputSection, SlashCommandResult,
 };
+use assistant_tool::ToolWorkingSet;
 use client::{self, proto, telemetry::Telemetry};
 use clock::ReplicaId;
 use collections::{HashMap, HashSet};