Support running partial graphql queries

[rmosolgo]

For ObjectCache, I could improve the granularity of caching if I could re-execute parts (eg, uncacheable parts) of queries without re-running the whole thing. This comes with some possible foot-guns, since fields sometimes depend on "upstream" context for their resolution, but it could also work pretty well in most cases.

I think the best API for me would be something like:

query = GraphQL::Query.new(...) # make a query somehow 
query_partial = query.partial_for("currentUser", "posts")
partial_result = query_partial.execute(@current_user.posts) # run the selected portion of the query using the given object as the underlying data

That would also support multiplexing, eg:

query = GraphQLQuery.new(...)
# Run these partials concurrently (sharing a dataloader cache, etc) 
results = query.execute_partials({
  ["currentUser", "posts"] => @current_user.posts, 
  ["currentUser", "profile", "auditLog"] => AuditLog.events(user: @current_user), 
  # Etc 
})

Then it's up to the caller how to handle those results.