Add reference types

[fersilva16]

This pull request introduces string references for GraphQL types, which aims to solve the problem with circular dependency between types in multiple files. Here's an usage example:

type: new GraphQLNonNull(new GraphQLList(new GraphQLNonNull('User')))

This PR has only the implementation for output types for now.

I've also made a RFC repository with a patch and pratical examples for testing: https://github.com/fersilva16/graphql-refs-rfc

Contributions and feedback are welcome.

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: fersilva16 / name: Fernando Silva (602fe1c88cd6002bb182a5a4a0569702a1ea811b)

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[daniloab]

How could we move forward with this pr?