rpm-ostree-backend doesn't support some valid ostree image reference formats

Originally reported to the ublue here: https://github.com/ublue-os/main/issues/132

OSTree native containers can be specified in a number of ways, and it appears that Discover only currently works if the image reference begins with ostree-unverified-registry. However, it may also start with a number of other things including ostree-unverified-image, ostree-image-signed, ostree-remote-image, ostree-remote-registry. When attempting to start Discover with ostree referencing an image that starts in one of these other ways, Discover currently crashes.

Example output when it crashes:

$ plasma-discover
rpm-ostree-backend: Invalid reference for OCI container ostree format: "ostree-unverified-image:docker://ghcr.io/ublue-os/kinoite-nvidia:38"
1pm-ostree-backend: Starting transaction to check for updates
rpm-ostree-backend: Error: Can not start a transaction for resource with an invalid format. Please file a bug.
org.kde.plasma.libdiscover: transaction not part of the model RpmOstreeTransaction(0x55a798d95d40)
KCrash: Application 'plasma-discover' crashing...
The Wayland connection experienced a fatal error: Bad file descriptor
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