main: Add support for overriding HTTP User-Agent I want this for https://github.com/bootc-dev/bootc/issues/1686 so we can distinguish pulls there. But more generally it's can be a good idea for people writing scripts using skopeo to set custom user agents so that registries can more easily trace which actors are performing tasks. Assisted-by: Claude Code Signed-off-by: Colin Walters <walters@verbum.org> - #2