fix: extendSchema for federation

[PacoDu]

This PR fixes an issue with extendSchema that wasn't working correctly with federated schema if we tried to extend the schema with an external type reference.

[PacoDu]

Do not merge !

service resolver sdl is not properly updated

[mcollina]

Any update on this?

[PacoDu]

We need a function to remove the federation metadata from the sdl (this is related to mercurius-js/mercurius-gateway#21). I tried to remove the directives from the schema before using printSchema but this also removes the directive statement from the type definition (which we need to keep for the sdl). I don't have the time to investigate this right now (I'm merging my schemas before as a workaround for now).

Also, I think this part is wrong: https://github.com/mcollina/fastify-gql/pull/262/files#diff-d9185daf57698ed90d572d1966e61b16R235

If anyone wants to fix the issue feel free to use this PR or start over. I'll try to fix this later if it's not resolved.

[mcollina]

Closing for now.