Support response: raw execution results

[fmigneault]

Description

Support the response: raw representation, which implies:

• Support response=raw and transmissionMode: value for a single output returns that output directly.
  see https://docs.ogc.org/is/18-062r2/18-062r2.html#req_core_process-execute-sync-raw-value-one

• Supporting the multipart/related format for multiple outputs, or simply the literal raw data format for individual outputs when requested as transmissionMode: value.
  see https://docs.ogc.org/is/18-062r2/18-062r2.html#req_core_process-execute-sync-raw-value-multi

• Reporting of those outputs should be done by Link headers only for requested outputs as transmissionMode: reference.
  see https://docs.ogc.org/is/18-062r2/18-062r2.html#req_core_process-execute-sync-raw-ref

• Mix of transmissionMode: value and transmissionMode: reference should do both of the previous, with the format being multipart/related or the literal media-type for remaining output(s) as value according to number of outputs not requested by reference. MUST use multipart/related with the contents provided for outputs requested by value, and Content-Location for outputs requested by reference.
  See https://docs.ogc.org/is/18-062r2/18-062r2.html#req_core_process-execute-sync-raw-mixed-multi

• All corresponding /req/core/process-execute-[execMode]-[transmissionMode]-[nOutputs]-[response] conformance items are to be added to /conformance endpoint when they get implemented.

• Potential replacement of response in body into return in Prefer header
  Support various return preference header #414

• Ensure that, even if the job was originally submitted with any response/Prefer value, the /jobs/{jobID}/results endpoint can be queried with a different ?response=raw|document or corresponding Prefer header to obtain the desired representation.

References

• https://docs.ogc.org/is/18-062r2/18-062r2.html#sc_execute_response (and subsequent sections with details for each case)
• https://docs.ogc.org/is/18-062r2/18-062r2.html#_response_7 (async extra details)
• must respect default resolution of Allow omitting execute outputs #375
• generation of the multipart contents: https://github.com/requests/toolbelt#multipartform-data-encoder
• relates to GD-535