Add an upload_targets field to the image request. This lets the API
caller specify multiple upload targets and upload options to be used.
If the upload target type does not match the upload options, the request
is invalid.
For backwards compatibility, the upload targets field is optional. If
it is not specified, the default upload target and upload options for
the image type are assumed, which is the same as the old behaviour.
Adding an explicit selection to the request makes it possible to support
multiple upload targets for the same image type. We plan to support
ostree commits being uploaded to both aws.s3 and pulp.
To report on the multiple upload requests, we add an upload_statuses
field to the ImageStatus response.
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