[4.0] Add a new status enum to ApolloQueryResult that describes the state of data

[jerelmiller]

#12333 did some work to ensure partial was properly reported. This PR also introduced a new complete flag, but after working through some of the tests, we noticed there was room for improvement, namely we don't have a great way to distinguish between when data is partial or when it is pending (i.e. a query with @defer that is still loading in chunks of data). As such, we plan to replace complete from #12333 with a new status enum that distinguishes between the different types.