zdb: dump ZAP_FLAG_UINT64_KEY ZAPs properly (#16334)

These are used for DDT and BRT stores. There's limited information
available to produce meaningful output, but at least we can put
something on screen rather than crashing.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.

Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>