Create an attribute field manager

We need an attribute field manager that can give us all attribute fields on a given variation type. It also needs to maintain a light map of attribute fields across bundles.
It should also have methods for creating or deleting attribute fields.

This common set of functionality is needed for the following issues:
#2689519: Allow product attributes to be assigned to product variation types
#2690681: Simplify the variation UX when each product has a single variation
#2690683: Rebuild the generated variation titles when an attribute value changes

Assigning to myself since I have it almost done.