postgres: introduce support for yaml files for backup-configs

[FrankEppink]

It would be helpful if backup-config also support yaml files, i.e. if
cloudctl postgres backup-config apply -f file.yaml
would be working analogue to
cloudctl postgres describe postgres1 -o yaml > postgres1.yaml and cloudctl postgres apply -f postgres1.yaml

The backup-config files could then be e.g. persisted in version control.