assistant: Add tool registry (#17331)

This PR adds a tool registry to hold tools that can be called by the
Assistant.

Currently we just have a `now` tool for retrieving the current datetime.

This is all behind the `assistant-tool-use` feature flag which currently
needs to be explicitly opted-in to in order for the LLM to see the
tools.

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -13,11 +13,13 @@ pub(crate) mod slash_command_picker;
 pub mod slash_command_settings;
 mod streaming_diff;
 mod terminal_inline_assistant;
+mod tools;
 mod workflow;
 
 pub use assistant_panel::{AssistantPanel, AssistantPanelEvent};
 use assistant_settings::AssistantSettings;
 use assistant_slash_command::SlashCommandRegistry;
+use assistant_tool::ToolRegistry;
 use client::{proto, Client};
 use command_palette_hooks::CommandPaletteFilter;
 pub use context::*;
@@ -214,6 +216,7 @@ pub fn init(
     prompt_library::init(cx);
     init_language_model_settings(cx);
     assistant_slash_command::init(cx);
+    assistant_tool::init(cx);
     assistant_panel::init(cx);
     context_servers::init(cx);
 
@@ -228,6 +231,7 @@ pub fn init(
     .map(Arc::new)
     .unwrap_or_else(|| Arc::new(prompts::PromptBuilder::new(None).unwrap()));
     register_slash_commands(Some(prompt_builder.clone()), cx);
+    register_tools(cx);
     inline_assistant::init(
         fs.clone(),
         prompt_builder.clone(),
@@ -401,6 +405,11 @@ fn update_slash_commands_from_settings(cx: &mut AppContext) {
     }
 }
 
+fn register_tools(cx: &mut AppContext) {
+    let tool_registry = ToolRegistry::global(cx);
+    tool_registry.register_tool(tools::now_tool::NowTool);
+}
+
 pub fn humanize_token_count(count: usize) -> String {
     match count {
         0..=999 => count.to_string(),