As with `volume export`, this was coded up exclusively in cmd/
instead of in libpod. Move it into Libpod, add a REST endpoint,
add bindings, and now everything talks using the ContainerEngine
wiring.
Also similar to `volume export` this also makes things work much
better with volumes that require mounting - we can now guarantee
they're actually mounted, instead of just hoping.
Includes some refactoring of `volume export` as well, to simplify
its implementation and ensure both Import and Export work with
readers/writers, as opposed to just files.
Fixes#26409
Signed-off-by: Matt Heon <mheon@redhat.com>