Photo picker folders - Githubissues

[x] My commits are rebased on the latest main branch
[x] My commits are in nice logical chunks
[x] My contribution is fully baked and is ready to be merged as is
[x] I have tested my contribution on these devices:
- 2018 iPad Pro 11-inch, iOS 16.4.1

Adds support for photo album folders (PHCollectionList) in the photo collection picker.

Renamed the existing PhotoCollection to PhotoAlbum to differentiate it from the new PhotoCollectionFolder.
- I reused "PHOTO_PICKER_UNNAMED_COLLECTION" as a fallback localized title for PhotoCollectionFolder, but I could also update it to its own string if desired.
Created a new PhotoCollection type that can hold either a PhotoAlbum or PhotoCollectionFolder.
Show 2x2 photo grid for collection folders.
- I made the folder previews match the behavior of the Photos app by showing the first item of the first four albums in a folder. The existing collection picker does, however, show the last item of each regular album instead of the first, so I could also update this PR to instead make it show the last item of the first four albums.
Allow PhotoCollectionPickerController to navigate into folders, pushing a new instance with its folder property set.

Folder from the main list	Folder contents

signalapp / Signal-iOS