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

include/sys/ddt.h

@@ -405,6 +405,9 @@ extern int ddt_walk(spa_t *spa, ddt_bookmark_t *ddb,
 
 extern boolean_t ddt_addref(spa_t *spa, const blkptr_t *bp);
 
+extern int ddt_prune_unique_entries(spa_t *spa, zpool_ddt_prune_unit_t unit,
+    uint64_t amount);
+
 #ifdef	__cplusplus
 }
 #endif