add support for dual gzip/zstd image support

[bsherman]

placeholder to match/support efforts ongoing in main repo.

[castrojo]

tldr the version of rpm-ostree in F39 is too old and will error out when it sees a zstd compressed image.

p5 is in progress of adding support to our workflows to push both gzip and zstd to the same image. Fedora will publish zstd:chunked by default for F41, but they are just now switching to OCI so they don't have previous users to transition. Bluefin will push both gzip/zstd in F40 to slurp up upgrading users from GTS, and then F41 will be zstd:chunked only.

We should encourage zstd:chunked as the default, but should probably have the option to push to both with comments so people don't end up in a situation where their older custom image can't upgrade.