Improve collection supplementary view support

[jessesquires]

CollectionViewModel.SectionModel is unnecessarily limited to:

let cellViewModels: [CollectionViewCellViewModel]?
let headerViewModel: CollectionViewSupplementaryViewModel?
let footerViewModel: CollectionViewSupplementaryViewModel?

CollectionViews can have arbitrary supplementary views — they are not limited to headers and footers.

(Note: FlowLayout is limited to only headers / footers)

We should update the model to reflect this.

[asmallteapot]

(Note: These are called CollectionSectionViewModel on master.)

There are two possible approaches for resolving this: we can keep CollectionSectionViewModel as a struct and try to make its API generic for any kind of supplementary view/collection view layout, or we can extract a CollectionSectionViewModel protocol and rename the current implementation to CollectionFlowLayoutSectionViewModel or something. Thoughts?

[jessesquires]

hey @asmallteapot! 👋 sorry for the delay 😳

I think my original comment was confusing. The problematic part is:

let headerViewModel: CollectionViewSupplementaryViewModel?
let footerViewModel: CollectionViewSupplementaryViewModel?

This should really just be:

let supplementaryViewModels: [CollectionViewSupplementaryViewModel]

To reflect that collection view has arbitrary supplementary views.