Add support for storing channel backups for multiple seeds on Google Drive and iCloud

[hsjoberg]

Can be separated by a hash generated by the seed

[Perlover]

+1

Very important! Without this, it is simply dangerous for one user to use multiple wallets on two or more devices!

It's should be related with this requested feature: https://github.com/hsjoberg/blixt-wallet/issues/366 and https://github.com/hsjoberg/blixt-wallet/issues/1032