Federation for existing schemas: graphql-transform-federation

Sometimes a rebuild to use graphql federation is not possible / desirable. I'm working on a schema transform that allows you to add federation to any existing schema: [graphql-transform-federation](https://github.com/0xR/graphql-transform-federation). The idea is that you add the information required for federation and the transform will ensure the schema conforms to the federation API.

I was wondering whether this approach aligns to with your ideas.

1. Are you interested in adding something like this to `buildFederatedSchema`? `buildFederationSchema(schema, config?: FederationConfig)`
2. What do you think of this approach compared to apollographql/apollo-server#3013 that relies on adding non-standard functions to GraphQL types

Other issues that could be solved with this:
https://github.com/prisma-labs/nexus/issues/148
https://github.com/MichalLytek/type-graphql/issues/351

Start by reviewing the proposed graphql-transform-federation project and the existing buildFederatedSchema API, then compare its approach with apollographql/apollo-server#3013. Done would require an agreed design for applying federation to existing schemas and a maintainer decision on whether to support it.

Written by the indexing model from the issue text.