Make it possible to add custom `init_options`

custom init options would help to add options like --append-only, --storage-quote QUOTA and other future options

https://borgbackup.readthedocs.io/en/stable/usage/init.html https://0xacab.org/riseuplabs/backupninja/blob/f9529d1d3f3714c707f38b8e44ac72a69af5483b/handlers/borg.in#L85