[4.0] Align ApolloQueryResult type and emitted result from the core API

[jerelmiller]

See #12327 as an example.

There are several cases where our TypeScript types and runtime behavior don't align. For example, the ApolloQueryResult type expects data to be a non-optional TData type. This is however incorrect as ObservableQuery will emit results where data is undefined or partial.

To ensure a good end-user experience, we'd like to make the type robust enough that data is a DeepPartial<TData> only when partial is set to true.