[9.18] fix: dev: Use DNS_RDATACOMMON_INIT to hide branch differences

Initialization of the common members of rdata type structures varies across branches. Standardize it by using the `DNS_RDATACOMMON_INIT` macro for all types, so that new types are more likely to use it, and hence backport more cleanly.

Closes #5467

Merge branch '5467-use-dns_rdatacommon_init-to-hide-branch-differences-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10833