Add the option of forwarding federated queries

For a development scenario it would be convenient if the federation service could act as a middle man for other underlying federation services. The idea is that a developer could setup the following

1. A Federation Gateway on their system
2. One of the underlying services on their system

The gateway would then hit a pre-existing gateway containing the rest of the graph, overriding the service that they are running locally. This would allow developer to not need to run the full suite of services on their machine in order to work on one. This would also need to be an opt-in flag due to the potential security implications of this setup.

Start by tracing the federation gateway’s handling of incoming federated queries and how configuration flags are defined; the issue names no files or tests. Clarify the opt-in forwarding flow between the local gateway, underlying service, and pre-existing gateway, then define tests for forwarding, the default-disabled behavior, and the security boundary.

Written by the indexing model from the issue text.