mbstream is already supported as a format name after MDEV-24580,
but additional code refactoring has been done to correctly display
the format name in log files and to check if the mbstream utility
is in the path. Also, for xtrabackup-v2 (only available in the 10.2)
both utilities are supported - both xbstram and mbstream, since they
are interchangeable in this context. In this case, the original
innobackupex always receives the correct --stream=xbstream option
as input, but the user can actually try to use the mbstream utility
during the transfer (if the user explicitly specifies this in the
configuration file).