Federation: strip common directives from federate services schema

[paolochiodi]

While working on mercurius-js/mercurius#227 and mercurius-js/mercurius#184 I have noticed that the fastify-gql implementation is lacking at least two features from Apollo server.

One of them is that apollo server strips common and type system directives definitions (i.e the definition of key directive) from federated services sdl before combining them.

As stated in their documentation this allows the federated service to provide the schema unchanged without needing to strip them out.

Documentation: https://www.apollographql.com/docs/apollo-server/federation/gateway/#type-system-directives
Apollo's code:
https://github.com/apollographql/apollo-server/blob/1c58ea3cbf6f5ead1afd5e81a1113a75441ead26/packages/apollo-federation/src/composition/composeAndValidate.ts#L16
https://github.com/apollographql/apollo-server/blob/main/packages/apollo-federation/src/composition/normalize.ts#L20
https://github.com/apollographql/apollo-server/blob/main/packages/apollo-federation/src/composition/normalize.ts#L270

[paolochiodi]

Tests: https://github.com/apollographql/apollo-server/blob/main/packages/apollo-federation/src/composition/__tests__/normalize.test.ts#L147