[Federation] Pass existing GraphQLSchema to buildFederatedSchema.

Just bringing it to the issues area to give it a few eye-balls, feel free to close if it's not wanted or correct.

https://github.com/apollographql/apollo-server/pull/3013

This PR adds support to decorate existing schemas with federation add-ons. While working on the implementation of `federation` in `type-graphql` (https://github.com/19majkel94/type-graphql/pull/369), I realized that it's sort of a pain and pointless to convert from `GraphQLSchema` to SDL and back and to SDL again as well as pluck out the `GraphQL*Type` resolvers when `buildFederatedSchema` should just be able to take in an already constructed schema and modify it.

Start with buildFederatedSchema and review PR 3013, which describes the proposed support for decorating an existing GraphQLSchema. Trace how the current federation implementation handles schemas, SDL, and GraphQL type resolvers. Done means an existing GraphQLSchema can be passed directly without the described SDL conversion and resolver extraction.

Written by the indexing model from the issue text.