fix: dev: Simplify the DNS_R_UNCHANGED handling in dns_resolver unit

Instead of catching the DNS_R_UNCHANGED from dns_db_addrdataset() (via
cache_rrset() and dns_ncache_add()) individually, mask it properly as
soon as possible by moving the sigrdataset caching logic inside the
cache_rrset() and returning ISC_R_SUCCESS from cache_rrset() and
dns_ncache_add() when the database was unchanged.

Closes #5473

Merge branch '5473-fix-crash-in-validated' into 'main'

See merge request isc-projects/bind9!10850