feat(): mercurius federation subscriptions

[Davide-Gheri]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: #2023

What is the new behavior?

Subscriptions are now correctly handled for Mercurius federated services

Does this PR introduce a breaking change?

• Yes
• No

Other information

[Davide-Gheri]

@kamilmysliwiec since the problem fixed in #2020 occurs only when the Mercurius Federation driver is used, should we move that workaround in the buildMercuriusFederatedSchema function introduced here?

[kamilmysliwiec]

@kamilmysliwiec since the problem fixed in #2020 occurs only when the Mercurius Federation driver is used, should we move that workaround in the buildMercuriusFederatedSchema function introduced here?

@Davide-Gheri makes sense! Let me know when it's updated & ready for review :)

And thanks for adding tests!

[Davide-Gheri]

@Davide-Gheri makes sense! Let me know when it's updated & ready for review :)

@kamilmysliwiec done!

[kamilmysliwiec]

lgtm

[smolinari]

@Davide-Gheri - Once more. I personally appreciate your efforts here with Nest's GraphQL. 👍 Awesome stuff!

Scott

[all2pie]

@Davide-Gheri Can you please also add it in docs

[WildEgor]

@Davide-Gheri can you please provide more info on how to use subs with federated graphql service via a gateway?