Allow exporting layers for a given ref

[volaya]

The plugin should allow exporting layer as a normal static layer (so changes wont be commited and the layer wont be linked to the repo) for any commit/tag/branch.

This requires Kart being able to tell the layer list at any ref, which is something that now cannot do, AFAIK, since it only list the repo content at the current HEAD, and does not allow any ref argument in the command.

[hamishcampbell]

Note that Kart can list datasets at a particular ref by providing the ref as an arg, e.g.

kart data ls some-other-branch
kart data ls 8ff9baf609f

Also worth noting that as of Kart v0.15.2 (https://github.com/koordinates/kart/releases/tag/v0.15.2) the CLI supports export so saving to a normal static layer form any refish should now be straight-forward.

etc