Add subscription support to @apollo/subgraph

[martijnwalraven]

The changes needed to support subscriptions in @apollo/subgraph are fairly minimal: we need to augment the resolver typings, and propagate the subscribe property to GraphQLFieldConfig.

One thing to call out is that this PR follows the definitions in graphql-js. That means both resolve and subscribe are optional, even though it could make sense to require at most once. The property types are also rather loosely defined: there is no attempt to validate that subscribe returns an AsyncIterator for example.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 22e8305

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

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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[trevor-scheer]

The code changes seem reasonable to me. Would you mind adding a test case and a changeset (via npx changeset)?

[hwillson]

Thanks for reviewing @trevor-scheer!

@martijnwalraven it sounds like this is really close - see Trevor's review comments. Thanks!

[hwillson]

👋 @martijnwalraven @trevor-scheer - is there anything left to do with this, or do you think we can go ahead and merge it?

[trevor-scheer]

I pushed up a test and changeset so this is ready to go. Validated that the test fails before @martijnwalraven's changes (and it clearly passes with them).