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/feature_flags/src/feature_flags.rs

@@ -49,6 +49,16 @@ impl FeatureFlag for Assistant2FeatureFlag {
     }
 }
 
+pub struct ToolUseFeatureFlag;
+
+impl FeatureFlag for ToolUseFeatureFlag {
+    const NAME: &'static str = "assistant-tool-use";
+
+    fn enabled_for_staff() -> bool {
+        false
+    }
+}
+
 pub struct Remoting {}
 impl FeatureFlag for Remoting {
     const NAME: &'static str = "remoting";