Creates a GCP private offer in Tackle.

When createInMarketplace is true:

• 201: Offer created in Tackle and, when requested, submitted to
  Google Cloud Marketplace. Transient marketplace failures after the
  Tackle draft is saved also return 201 with marketplaceErrors and
  workflow.status=failed. Retry with the returned id or
  POST …/publish.
• 400: Validation failure, offer cannot be published, or Google Cloud
  Marketplace rejected the payload. Body is
  {"error":"<message>","id":"<poId>"} when a Tackle draft was already
  saved. Fix cloudData, then retry publish.
• Invalid marketplace offer data returns 400, not 201.

Amendments: There is no separate amend endpoint. To create an
amendment private offer, use this same create path and set
privateOffer.cloudData.singleProductOffer.amendedPrivateOffer to the
parent private-offer resource name. See PrivateOfferCloudData.