subgraph response parts aren't propagated

[garypen]

When the router calls a subgraph service, we take the response process the body and do some general validation to ensure the GraphQL looks right before returning the response back through the pipeline. The rest of the response is discarded. This means any headers returned from a subgraph don't make their way back into the response.

This is not a good outcome and it results in issues in the pipeline where plugin authors (in rhai or rust) expect to be able to interact with subgraph headers.