Resolve Promise references for interface types before handing off to resolveType fn

[trevor-scheer]

Fixes #2552

Commits in order first reproduce the issue then resolve it.

Interface entities are a relatively new thing, but I'm surprised this hasn't surfaced yet since it seems like it'd be a showstopper for anyone trying to use it. If my understanding of the issue is correct, this seems like a reasonable solution... though it's possible the issue is wrong.

FWIW, none of our __resolveReference functions in tests return Promises, it might be more realistic if they did.

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Latest commit: 5a1b8a2

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Name	Type
@apollo/subgraph	Patch
@apollo/federation-internals	Patch
@apollo/gateway	Patch
@apollo/composition	Patch
@apollo/query-planner	Patch
@apollo/query-graphs	Patch
apollo-federation-integration-testsuite	Patch

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[pcmanus]

I do think this is the issue and the fix looks good to me.

Testing wise, I'll note that there is few variants on how you can make this work: you can skip the __resolveType but return the __typename in the __resolveReference instead, or you can use isTypeOf for instance, and those variants are tested here, but indeed only for there synchronous versions. It might be nice to try to modify this (parameterised) test to include asynchronous variants of the various options. In particular, the added test in this patch tests an async __resolveReference but a sync __resolveType, but you can also have async __resolveType (with or without async __resolveReference), and while I believe the code is fine for those now, would be nice to cover everything.