feat(smith): add link directive to schema definition generation

[lrlna]

This add link directive to generated schema definition.

I run this with our API Schema fuzzer in apollo-rs-fuzz and it passes the "valid supergraph" validation. We now, however, run into the issue of apollo-smith not always generating semantically valid graphql, e.g. #425 #180. I'll start to minimally tweak those next.