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/context_tests.rs

@@ -1,6 +1,5 @@
 use super::{AssistantEdit, MessageCacheMetadata};
 use crate::slash_command_working_set::SlashCommandWorkingSet;
-use crate::ToolWorkingSet;
 use crate::{
     assistant_panel, prompt_library, slash_command::file_command, AssistantEditKind, CacheStatus,
     Context, ContextEvent, ContextId, ContextOperation, InvokedSlashCommandId, MessageId,
@@ -11,6 +10,7 @@ use assistant_slash_command::{
     ArgumentCompletion, SlashCommand, SlashCommandContent, SlashCommandEvent, SlashCommandOutput,
     SlashCommandOutputSection, SlashCommandRegistry, SlashCommandResult,
 };
+use assistant_tool::ToolWorkingSet;
 use collections::{HashMap, HashSet};
 use fs::FakeFs;
 use futures::{