Bring `CustomCellProvider` to the HeaderView

[MosheBerman]

The new CKCustomCellProvider API works well for cells, but custom cells can look funny beneath a mismatching header. The cell provider API is still useful for customizing cell content, but unless you can get your header working the way you'd like, it's a little less useful. UIAppearance works, but is far from idea.

Some things I'd like to be able to provide "for free" while still allowing maximum customization:

• Automatic column name and month name label layout
• Improved Accessibility (#122) , including VoiceOver and Preferred Content Size.

Ideally, this would help support #120 (Add A Modern Calendar Scrolling Mode.)