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/store/captain/copilotThreads.js

@@ -0,0 +1,7 @@
+import CopilotThreadsAPI from 'dashboard/api/captain/copilotThreads';
+import { createStore } from './storeFactory';
+
+export default createStore({
+  name: 'CopilotThreads',
+  API: CopilotThreadsAPI,
+});