Add ability to transform an existing GraphQLSchema to a federated schema

[michael-watson]

This brings over the work from a previous Apollo Server PR

There are current GraphQL servers that have patterns that create a GraphQLSchema from a pattern defined by the authors of that server.

For example, RedwoodJS uses makeMergedSchema to create a GraphQLSchema from a set of sdl blobs and defined services in the projects structure. There is no way to introduce the Apollo Federation Subgraph Spec into the schema and these servers require someway to modify their existing GraphQLSchema to add the Apollo Federation pieces.

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