Added empty definition for Subscription when composing the schema

[jsangilve]

Adds an Empty Definition for Subscription type, the same way it's already done for Query and Mutation.

Solves #3357

[apollo-cla]

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[jsangilve]

Can you folks finish the review of this? It's breaks the Gateway when one of the federated services have subscriptions.

Needs tests.

[abernix]

@jsangilve Do you mind adding some tests for this using the original reproduction you opened in #3357?

Please understand that we have other priorities as well and there are a lot of other things in motion and I'm not the only person who has to review and think about this. Thanks!

[abernix]

I appreciate that you're exploring this but, upon further consideration, let's re-open this after we've discussed the use cases here a bit more in the issue that you opened. I'll comment over there.

[ColorBuffer]

Hi,
I also need this feature as a quick fix to have subscriptions in a "single API".

thanks for taking the time to review.

[cletustboone]

Is forking the only option here? I am running up against this issue as well.