feat(x86_64_v2): targetuserspacecreator: support custom rootfs for target userspace bootstrap

[ykohut]

Add the ability to use a pre-built rootfs instead of bootstrapping the target userspace container via 'dnf install --installroot'. This enables 9->10 x86_64_v2 upgrades.

Two new methods are supported, checked in order of priority:

1. Local tarball: place a .tar.xz/.tar.gz/.tar.bz2/.tar archive in /etc/leapp/files/rootfs/ and it will be extracted directly.

2. Container image: set LEAPP_DEVEL_ROOTFS_CONTAINER_IMAGE to a container image URL (e.g. quay.io/almalinuxorg/almalinux:10). The image is pulled via podman, exported as a flat filesystem, and extracted into the userspace directory. The platform can be specified via LEAPP_DEVEL_ROOTFS_CONTAINER_PLATFORM (e.g. linux/amd64/v2).

If neither is configured, the original DNF-based bootstrap is used.

Also fix the update-ca-trust invocation in _prep_repository_access() to call the binary directly instead of wrapping it with 'su', which may not be present in minimal rootfs images.

[github-actions]

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