Test this library against apollo-federation-subgraph-compatibility

[rvanlaak]

I see this library has Federation 2 support, which is something that other libraries are struggling with:

• Implement Apollo Federation v2 Skillshare/apollo-federation-php#26
• Add extensibility for easy integration of Apollo Federation PHP (v2) overblog/GraphQLBundle#1040
• GraphQL Federation support webonyx/graphql-php#515

As this library (up to my research) seems to be the only PHP one that provides Federation 2 support, what about testing that support against the schemas and making sure that this library thereafter gets the visibility it deserves by adding it to the GraphQL documentation pages?

Testing the implementation should be possible by adding it to
https://github.com/apollographql/apollo-federation-subgraph-compatibility/tree/main/implementations/php
... an example on how compatibility was tested by Skillshare can be found at Skillshare/apollo-federation-subgraph-compatibility#1