Allow completing slash command arguments from extensions (#13240)

This PR extends the extension API with support for completing slash
command arguments for slash commands defined in extensions.

Release Notes:

- N/A

crates/assistant/src/slash_command/fetch_command.rs

@@ -113,7 +113,7 @@ impl SlashCommand for FetchSlashCommand {
     }
 
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         _query: String,
         _cancel: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,