[DataSource] implement prefetching

[jessesquires]

Discussion

Initial thoughts:

• Create a new DataPrefetchAdapter class that conforms to UICollectionViewDataSourcePrefetching
  • This should map the prefetch API index paths to identifiers
  • Tell the client "preload these identifiers"
  • Maybe have clients pass in closures or something?
• Pass this object into CollectionViewDriver and hook it up to the collectionView.prefetchDataSource
• Allow client to provide a prefetching queue

Potential problems

• CollectionViewModel is designed to be stateless, so maybe this just doesn't make sense?
• When prefetching operations complete, we need a way to say "ok, now update the collection view."
• Would this involve needed to regenerate the CollectionViewModel? Would CellViewModel instances be mutable?

Docs

• https://developer.apple.com/documentation/uikit/uicollectionviewdatasourceprefetching
• https://developer.apple.com/documentation/uikit/uicollectionview/1771768-prefetchdatasource

Sample Code

• Prefetching collection view data
• Building High-Performance Lists and Collection Views

This sample code is pretty interesting. They use NSOperationQueue or DispatchQueue. There's lots of boilerplate there that we should attempt to encapsulate. Clients can give us a queue, or we can maintain our own.