[RFE] Improve/unify bootc source and target imgref handling

[jikortus]

The issue below has been copied from downstream https://issues.redhat.com/browse/RHEL-162331 and is related to using Anaconda to deploy the bootable images:

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As we recently found out, if the user uses Anaconda to install a system via the kickstart bootc command just with the --source-imgref option, such as bootc --source-imgref=registry:URL-to-image, the installation is successful, but the target reference for bootc doesn't work (e.g. for bootc update). The reason is that bootc fails to understand the registry: transport prefix for target image reference, but at the same time the prefix is required for source image reference and Anaconda sets the --target-imgref to the same value as --source-imgref by default.

This makes the user experience rather confusing and I believe we should unify how the source/target references are handled. As of now, the only way to work around this is to use bootc with --source-imgref=<transport>:<URL-to-image> supplemented by --target-imgref=<URL-to-image>.

There may be at least the following options to make the experience consistent:

1. Make bootc autodetect the transport (e.g. registry:) from the URL.
2. Add a --source-transport option to second the existing --target-transport counterpart and make Anaconda use it implicitly even when not specified by the user.
3. Make anaconda split the target transport and target image reference (even for the default taken from the source reference) and pass those separately to bootc via the already existing respective options.
4. Combinations of the previous options.

Personally, when trying to see the problem by user's eyes, especially when the user is new to the Image Mode, I'd prefer option 1. This would mean that:

• It's possible to only pass the source reference to the bootc command.
• The format of the option values passed using --source-imgref and --target-imgref is consistent with the way bootc is operated directly by the user on the command line.
• If extended by option 2., also the way the source and target references are specified is consistent, enhancing the UX even more. On the other hand, it would mean that Anaconda needs to get some more logic to handle the new option vs. transports specified by the prefixes + handling a new option for bootc.