Support reports format and long-running jobs

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Related: gbv/data-validation-report-format#7 and https://gbv.github.io/data-validation-report-format/#reports

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Return with HTTP Status Code 201 and Header Location with URL of current report and an optional access token. Reports may be deleted and validation may be aborted with access token. See https://stackoverflow.com/a/47157391 for details of token header.

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• When to keep reports?
  • always/never
  • pass whether to store on validation request
  • after some timeout
  • ...?
• How long to keep reports?