rgw: add rgw_enable_dedup_threads config to control per-RGW dedup participation

Add rgw_enable_dedup_threads (default true) and rgw_nfs_run_dedup_threads
(default false) config options to control whether a given RGW instance
participates in dedup background processing.

This follows the same pattern used by rgw_enable_gc_threads,
rgw_enable_lc_threads, and rgw_enable_restore_threads. The config is
evaluated at startup in init_dedup(); when disabled, the dedup Background
object is not created, the thread is not spawned, and the instance does
not register a RADOS watch on the dedup control object.

Disabled RGW instances are invisible to the dedup cluster coordination
mechanism -- they receive no notifications, cause no timeouts, and do
not grab shard tokens.

Signed-off-by: Gabriel BenHanokh <gbenhano@redhat.com>
Assisted-by: Cursor Opus-4.6 <cursoragent@cursor.com>