Add Apollo federation support to GraphQL implementation

[renepardon]

• Added or updated tests
• Added Docs for all relevant versions
• Updated CHANGELOG.md

Resolves #911

Changes

Implementation of federation specification as described here:

• https://www.apollographql.com/docs/apollo-server/federation/introduction/
• https://www.apollographql.com/docs/apollo-server/federation/federation-spec/

Breaking changes

[renepardon]

Currently blocked by webonyx/graphql-php#552 as it is not an option to copy the SchemaPrinter into lighthouse just to "extend" it.

[jerredhurst]

@spawnia - Is there a way to help in webonyx/graphql-php#552 so this can keep moving forward?

[spawnia]

@jerredhurst my favorite approach would be to add an option to their printer to print directives.

If that is not possible, we can go ahead and implement the simplest possible approach to solve our specific use case. Maybe we can find a way to make the SchemaPrinter extensible enough so we can reuse some of its logic?

[jerredhurst]

@spawnia - Thank you for the response. In regards to your comment:

my favorite approach would be to add an option to their printer to print directives.

Have you implemented this in other cases? Is there something we can see?

[spawnia]

@renepardon thanks for the effort, too bad you moved on.

Closing this PR, i pulled in the changes to a branch: https://github.com/nuwave/lighthouse/tree/federation