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

Cargo.lock

@@ -373,6 +373,7 @@ dependencies = [
  "anyhow",
  "assets",
  "assistant_slash_command",
+ "assistant_tool",
  "async-watch",
  "cargo_toml",
  "chrono",
@@ -454,6 +455,20 @@ dependencies = [
  "workspace",
 ]
 
+[[package]]
+name = "assistant_tool"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "collections",
+ "derive_more",
+ "gpui",
+ "parking_lot",
+ "serde",
+ "serde_json",
+ "workspace",
+]
+
 [[package]]
 name = "async-attributes"
 version = "1.1.2"