assistant2: Add support for using tools (#21190)

This PR adds rudimentary support for using tools to `assistant2`. There
are currently no visual affordances for tool use.

This is gated behind the `assistant-tool-use` feature flag.

<img width="1079" alt="Screenshot 2024-11-25 at 7 21 31 PM"
src="https://github.com/user-attachments/assets/64d6ca29-c592-4474-8e9d-c344f855bc63">

Release Notes:

- N/A

crates/assistant_tools/src/now_tool.rs

@@ -30,7 +30,7 @@ impl Tool for NowTool {
     }
 
     fn description(&self) -> String {
-        "Returns the current datetime in RFC 3339 format.".into()
+        "Returns the current datetime in RFC 3339 format. Only use this tool when the user specifically asks for it or the current task would benefit from knowing the current datetime.".into()
     }
 
     fn input_schema(&self) -> serde_json::Value {