rgw/pubsub: fix uninitialized num_shards causing topic deletion hang

rgw/pubsub: fix uninitialized num_shards causing topic deletion hang

The num_shards member of rgw_pubsub_dest was not included in JSON
serialization (dump/decode_json), causing garbage values when topic
metadata synced between zones. This resulted in topic deletion
iterating millions of times over non-existent shards, blocking
frontend pause during realm reload for extended periods.

Fixes: https://tracker.ceph.com/issues/75466
Signed-off-by: Oguzhan Ozmen <oozmen@bloomberg.net>