Support for DOA RRs along with relevant system test extensions is in the
rt45612 branch, please review.

Unit tests will be added after #45610 gets merged, to avoid further code
duplication in lib/dns/tests/rdata_test.c (unless #45610 takes too much
time to review, in which case I will add unit tests for DOA RRs to
rdata_test.c in its current form).