Allow repository to be split between multiple, partially offline, media

[skirmess]

It would be nice if rustic could split a repository onto multiple external disks, which would take the hot/cold use case even further. And it would allow us to backup more data than what fits on an external disk. Here is how I would like to use it:

• Create a repository on an external USB disk and backup your data (or part of you data if the disk is to small) to the disk

• Remove the disk from your system and move it to an external location.

• Take a new USB disk and create a repository on it. This repository will be an extension to the other one.

• Do an incremental backup to that disk. (The backup is incremental to the data on the currently not available disk)

• Remove the disk from your system and move it to an external location

And so on until you, after some time, cycle back to the initial disk. Then rustic would prune no longer needed data on that disk and add the new incremental data.

The only time you would need access to all the disks is in case of recovery.

This could even be improved if it would allow us to specify to have all data on at least two different disks.