Send response errors only to coprocessor

[smyrick]

Is your feature request related to a problem? Please describe.**

Today I can configure the Router to send the response, which might have data or errors or extensions, to the coprocessor so that it can perform extra logic. However I can only choose to send the whole response body. If I am returning data back this body may be quite large and can cause performance issues or even errors inside my coprocessor trying to process large responses.

Instead I would like the ability to decide what parts of the GraphQL response I would like sent to a coprocessor.

Config today

coprocessor:
  url: http://127.0.0.1:8081
  supergraph:
    response:
      body: true

Describe the solution you'd like**

I would like the ability to decide what parts of the GraphQL response I would like sent to a coprocessor, so I can reduce the payload size.

It probably makes sense for this to be at the SupergraphResponse phase since that is where the Request is also in a parsed state vs the RouterResponse should be reserved for changing the entire response as it is about to be sent via HTTP back to the client

Note: We should include support for data, errors, and extensions separately

Possible new config:

coprocessor:
  url: http://127.0.0.1:8081
  supergraph:
    response:
      body_data: false
      body_errors: true
      body_extensions: true

Describe alternatives you've considered**

I could use Rhai scripts to process particular parts of the response

Additional context

Actually having a large payload can cause the coprocessor to fail which is a separate issue, so this feature request is one way in which we can solve that