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_tool/src/assistant_tool.rs

@@ -0,0 +1,35 @@
+mod tool_registry;
+
+use std::sync::Arc;
+
+use anyhow::Result;
+use gpui::{AppContext, Task, WeakView, WindowContext};
+use workspace::Workspace;
+
+pub use tool_registry::*;
+
+pub fn init(cx: &mut AppContext) {
+    ToolRegistry::default_global(cx);
+}
+
+/// A tool that can be used by a language model.
+pub trait Tool: 'static + Send + Sync {
+    /// Returns the name of the tool.
+    fn name(&self) -> String;
+
+    /// Returns the description of the tool.
+    fn description(&self) -> String;
+
+    /// Returns the JSON schema that describes the tool's input.
+    fn input_schema(&self) -> serde_json::Value {
+        serde_json::Value::Object(serde_json::Map::default())
+    }
+
+    /// Runs the tool with the provided input.
+    fn run(
+        self: Arc<Self>,
+        input: serde_json::Value,
+        workspace: WeakView<Workspace>,
+        cx: &mut WindowContext,
+    ) -> Task<Result<String>>;
+}