fix: extendSchema does not work with federationMedata set to true

[PacoDu]

The _service { sdl } is not updated on extendSchema with federationMetadata: true, which means that extendSchema does not work at all if federation is enabled because the sdl does not reflect the changes.

My current workaround is to merge schemas and resolvers from different module before registering mercurius. But that makes it difficult to split code in different modules without some custom wrapper.

IMO we should clearly state in the documentation that extendSchema does not work with federation and throw if used or better fix the issue.

I think this is related to mercurius-js/mercurius-gateway#23 mercurius-js/mercurius-gateway#21 and my first attempt to fix this #262

I'll try an other fix by implementing stripCommonPrimitives based on apollo implementation as suggested in mercurius-js/mercurius-gateway#21. This should simplify the logic of buildFederatedSchema and extendFederatedSchema proposed in #262

[mcollina]

I think the correct path is to throw in extendSchema if federation is enabled and then have a better fix that adds support for it.

[simoneb]

Any news on this one?