Support assigning serializer instances to views

[stevelacey]

Support assigning serializer instances to views as well as types.

This saves on needing to create types for simpler views that just need to limit the fields dumped:

class CandidateList(ListAPI):
    serializer = CandidateSerializer(
        only=[
            "freelancer.id",
            "freelancer.rating",
            "freelancer.user",
            "job.id",
            "job.company.id",
            "job.company.name",
            "job.title",
            "job.closes_at",
            "status",
        ]
    )

Checklist

• This PR increases test coverage
• This PR includes README updates reflecting any new features/improvements to the framework