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

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/