Provide an API to watch fragments

[jpvajda]

Relates to

apollographql/apollo-client#8236
apollographql/apollo-feature-requests#198

Details

This relates to our work in our webClient to offer a useFragment hook, and there has been some interest in this same level of functionality in Kotlin.

useFragment is a read-only reactive/live binding into the cache, providing an always-up-to-date view of whatever data the cache currently contains for a given fragment.

It makes sense to want to watch just a fragment but it's not possible in Kotlin at the moment; it looks like our watcher functions are restricted to queries only.

[jpvajda]

From a discussion I had with the requester of this feature:

There is sort of a work around where you can create a getFoo(uuid: ID!): Foo query then have the cache know how to lookup that entity even if it came in a list. But would be nice to have it as a first class concept and it seems like it could be possible given things are already effectively stored this way in the normalized cache.