Use ostree.ImageOptions for the request parameters instead of a
SourceSpec on the imageRequest.
When preparing the image request, add the ostree values from the API's
compose request to the ostree options on the image options of the image
request.
It's not necessary to create a source spec and it's also not necessary
to add the default ref when it's not specified in the request for an
ostree-based image type. Both of these will be handled by the Manifest
generation based on the ostree options (imageOptions.OSTree). The image
functions will take care of setting any missing parameters or returning
errors if any required parameters are missing.
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