chg: dev: Change NSEC3 and NSEC3PARAM rdata struct fields to use isc_region_t

Replace the separate pointer+length field pairs in the NSEC3 and NSEC3PARAM rdata structures (salt/salt_length, next/next_length, typebits/len) with isc_region_t, making the fields self-describing and eliminating a class of length-mismatch bugs.

Merge branch 'ondrej/change-nsec3-and-nsec3param-to-use-isc_region_t' into 'main'

See merge request isc-projects/bind9!11592