Implement `FetchedResultsCoordinator` a wrapper for `FetchedResultsController`/`Delegate`

[jessesquires]

The goal is to remove all boilerplate with FRC.

The FetchedResultsCoordinator will:

• create and own the NSFetchedResultsController
• be the controller's delegate, implement NSFetchedResultsControllerDelegate
• be the dataSource via DiffableDataSource APIs
• UICollectionView only

Todo

• [x] implement initial FetchedResultsCoordinator (get working with configuring cells)
• [x] update example project
• [x] add support for supplementary views
• [ ] cell interaction: didSelect, etc.
• [ ] cell swipe actions (delete, favorite)
• [ ] data source prefetching?
• [ ] update guides
• [ ] add docs
• [ ] add tests

Notes & docs:

• https://www.avanderlee.com/swift/diffable-data-sources-core-data/