add RESTIC_PASSWORD_COMMAND

[ColCh]

Hello!

First of all, thank you for this program.

I'm using keyring to store password for restic repo https://pypi.org/project/keyring/

in restic, it's used as RESTIC_PASSWORD_COMMAND env var

but the problem is, in runrestic json schema command is not defined as a valid value

https://github.com/sinnwerkstatt/runrestic/blob/6c10fedc1814e760e8ec41c5d167463d3f9a16e1/runrestic/runrestic/schema.json#L46C1-L49C8

so, I'm forced to specify at least this RESTIC_PASSWORD_FILE=""

[andreasnuesslein]

hi @ColCh , thanks for the report. would you be able to create a PR?

[ColCh]

sure! I think I could do that :)