assistant2: Add configuration (#23481)

This PR wires up the ability to configure Assistant2.

<img width="1309" alt="Screenshot 2025-01-22 at 1 52 56 PM"
src="https://github.com/user-attachments/assets/3de47797-7959-47af-bd93-51f105e87c28"
/>

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -1,4 +1,5 @@
 mod active_thread;
+mod assistant_configuration;
 mod assistant_model_selector;
 mod assistant_panel;
 mod buffer_codegen;
@@ -41,6 +42,7 @@ actions!(
         RemoveAllContext,
         OpenHistory,
         OpenPromptEditorHistory,
+        OpenConfiguration,
         RemoveSelectedThread,
         Chat,
         ChatMode,