persist/push: Need more flexibility in input/response handling

[johnrix]

Mutation response handling seems to currently assume the result is just an instance of the object type being updated. This does not, for example, allow the flexibility to implement a model such as that proposed here by the Apollo team on how to structure mutations: https://blog.apollographql.com/designing-graphql-mutations-e09de826ed97

It would be useful for the Adapter to offer mechanisms to control the structuring of the inputs (see my other issue here for potential use case) and processing of the responses to mutations.