feat: Add stores for copilotMessages and copilotThreads (#11603)

- Set up stores for copilotThreads and copilotMessages.
- Add support for upsert messages to the copilotMessages store on
receiving ActionCable events.
- Implement support for the upsert option.

app/javascript/dashboard/api/captain/copilotThreads.js

@@ -0,0 +1,9 @@
+import ApiClient from '../ApiClient';
+
+class CopilotThreads extends ApiClient {
+  constructor() {
+    super('captain/copilot_threads', { accountScoped: true });
+  }
+}
+
+export default new CopilotThreads();