Closes #15087: Support for specifying a subset of API serializer fields

[jeremystretch]

Fixes: #15087

• Introduces support for specifying a subset of serializer fields to be returned in a REST API request (e.g. GET /api/dcim/sites/?fields=id,name,status,region)
• Introduces the get_prefetches_for_serializer() utility function
• Extends the base API viewset to automatically resolve the necessary queryset prefetches for a serializer based on the requested fields
• Removes all now-unnecessary explicit calls to prefetch_related() on API views (a few remain for various reasons)
• Removes the brief_prefetch_fields property from API views

[jeremystretch]

The test failure is due to a bug in the nested serializer for FHRP group assignments. (It should be returning group rather than group_id.)