[FE Offline Transfer] Determine state management and polling approach

The multi-tab design for the Export and Import flows indicate a use case for a shared state. At the time of writing, the assumption is that this research might be more practical once we've built the base multi-tab structure and perhaps the entity selection in #586918. The order of tickets might change though.

We need to determine which of GitLab's approved options would be best:

• Apollo
• Pinia

Bearing in mind the following considerations

• The backend has implemented a REST API #538940 (closed)
• Polling for transfer updates will be required. There has been some discussion about GQL subscriptions or websockets for this. #583817

Resources

https://docs.gitlab.com/development/fe_guide/state_management/

Acceptance criteria

• State manager is chosen and approved by team
• Created follow up issue (or issues) for building shared state for the multi-tab UIs.
• Documented how to use both GQL subscriptions and websockets given the choice of state manager