Add fromFirestore support

[danReynolds]

This PR introduces the fromFirestore API for filtering what fields returned from Firestore should be merged into the cache on the client. This is helpful if you've made cache writes on the client optimistically for fields you intend to persist to the server later, but in the meantime you need to refetch from the server.

Up until now, the assumption had been made that whatever comes back from the server is the source of truth and should overwrite what's in the cache. The fromFirestore API makes it possible to specify a custom merge function for when values come back from the server and to complements the toFirestore API that provides the opposite filtering of what values you want sent to the server.