[DataSource] implement section index titles

Docs: https://developer.apple.com/documentation/uikit/uicollectionviewdatasource#2851612

```swift
optional func indexTitles(for collectionView: UICollectionView) -> [String]?

optional func collectionView(_ collectionView: UICollectionView, indexPathForIndexTitle title: String, at index: Int) -> IndexPath
```

This will need to be implemented at the `CollectionViewModel` level.

Maybe we wrap this functionality in a new protocol, pass that in, then call these methods via the data source implementation.

Rough example:

```swift
protocol IndexTitleViewModel {

var indexTitles: [String]? { get }

func indexPathForIndexTitle(_ title: String, at index: Int) -> IndexPath
}
```