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/tools/now_tool.rs

@@ -0,0 +1,60 @@
+use std::sync::Arc;
+
+use anyhow::{anyhow, Result};
+use assistant_tool::Tool;
+use chrono::{Local, Utc};
+use gpui::{Task, WeakView, WindowContext};
+use schemars::JsonSchema;
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Serialize, Deserialize, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+pub enum Timezone {
+    /// Use UTC for the datetime.
+    Utc,
+    /// Use local time for the datetime.
+    Local,
+}
+
+#[derive(Debug, Serialize, Deserialize, JsonSchema)]
+pub struct FileToolInput {
+    /// The timezone to use for the datetime.
+    timezone: Timezone,
+}
+
+pub struct NowTool;
+
+impl Tool for NowTool {
+    fn name(&self) -> String {
+        "now".into()
+    }
+
+    fn description(&self) -> String {
+        "Returns the current datetime in RFC 3339 format.".into()
+    }
+
+    fn input_schema(&self) -> serde_json::Value {
+        let schema = schemars::schema_for!(FileToolInput);
+        serde_json::to_value(&schema).unwrap()
+    }
+
+    fn run(
+        self: Arc<Self>,
+        input: serde_json::Value,
+        _workspace: WeakView<workspace::Workspace>,
+        _cx: &mut WindowContext,
+    ) -> Task<Result<String>> {
+        let input: FileToolInput = match serde_json::from_value(input) {
+            Ok(input) => input,
+            Err(err) => return Task::ready(Err(anyhow!(err))),
+        };
+
+        let now = match input.timezone {
+            Timezone::Utc => Utc::now().to_rfc3339(),
+            Timezone::Local => Local::now().to_rfc3339(),
+        };
+        let text = format!("The current datetime is {now}.");
+
+        Task::ready(Ok(text))
+    }
+}