Make `rustic cat config` print default values being used

[simonsan]

Yes, you can use rustic cat config. This will, however, only show the raw json of the config file and not the default values used if an item is missing... If anyone is up with a PR to also show the values, we'll happily merge that!

Originally posted by @aawsome in https://github.com/rustic-rs/rustic/discussions/724#discussioncomment-7264887