mariabackup: create method for backup tool to extract from mbstream

As I mentioned in MDEV-15713, some people seem to think that the --stream option can be used like:

mariabackup --backup --stream=xbstream --user=root > backup.xbstream

cat backup.xbstream | mariabackup --prepare --stream=xbstream

The proper method to use it seems to be more like this:

mariabackup --backup --stream=xbstream --user=root > backup.xbstream

mbstream -x < backup.xbstream

mariabackup --prepare --target-dir=./

Some users think that mariabackup should have a way to extract the files from the mbstream archive itself without making the user go through the extra step.

Is that feasible and/or desirable?