non-UTF8 filename support

Since its creation ostree (current bootc backend) intentionally didn't support non-UTF8 filenames. In theory, we could lift that restriction as we look towards a direct composefs backend.

I personally think it's almost always going to be a mistake to ship non-UTF8 filenames in containers, but just filing this so we have a tracker.

Also, it'd probably be good to add this to `container lint`.

Review the existing ostree backend and the proposed direct composefs direction first; no implementation files or tests are identified in the issue. Clarify whether non-UTF8 filenames should be supported and how container lint should report them. Done means the support policy and lint behavior are defined and implemented with verification.

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