Validate and document approach for testing Federation with hosted services

[hwillson]

We're receiving more and more requests to validate Federation support with hosted services. This project is mostly validating Federation support with locally run applications/subgraphs (that are committed to the repo), but does support validating remote subgraphs (including those hosted in the cloud / behind an API). We're currently using an nginx based proxy approach with AppSync, that could be used with other providers. We should:

• Document this approach in the README
• Explain how custom headers like those needed for auth can be included
• Adjust the test results to clearly identify if they're the result of running against a local or remote subgraph