Support for GraphQL Federation?

**Description**
Since some time, Apollo supports a feature called Federation, which makes it possible to split a monolithic GraphQL server into microservices, so that each service only holds a small subgraph. The federation gateway stitches all these subgraphs back together into a single graph.

It would be great, if this kind of architecture would be possible with API platform, too?
I think, that it would be really beneficial for the PHP ecosystem to be listed in the supported libraries/frameworks, too (https://www.apollographql.com/docs/federation/other-servers/).

I think, that the biggest challenge for API platform would be, to support the "extension" of a graph entity, which isn't defined in the service itself (but another service).

Useful links:
* https://www.apollographql.com/docs/federation/
* https://docs.nestjs.com/graphql/federation