Add support for cold storage

[aawsome]

see restic/restic#3235 and restic/restic#3202

This will be tackled after support for cache #33 is merged.

[aawsome]

I will work on this with high priority as I use cold storage and the linked restic PR myself in productive repositories.

However supporting the hot/cold logic is just one step. Another is, of course, to support more backends, especially the ones which offer cold storages.