Add DDT prune command

Requires the new 'flat' physical data which has the start
time for a class entry.

The amount to prune can be based on a target percentage of
the unique entries or based on the age (i.e., every entry
older than N days).

Sponsored-by: Klara, Inc.
Sponsored-by: iXsystems, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Closes #16277