Add to Apollo subgraph compatibility test suite

[kdawgwilk]

Would be great to have compatibility determined by the new test suite from Apollo. I have a PR up adding an example subgraph and would love any feedback to improve it

apollographql/apollo-federation-subgraph-compatibility#42

[carldunham]

@kdawgwilk, how about just taking your implementations/gqlgen in that PR and dropping it into example/federation/products here? It won't pass until multiple @key directives are supported, of course.

[kdawgwilk]

I am okay with doing that but Apollo will likely want it in their test suite so we could have it in both places