[dns] OTBR DNS server implementation of SOA record for its authoritative zone (default.service.arpa)

[EskoDijk]

Describe the bug

For a DNS authoritative server, the SOA and NS records are mandatory (Section 6.1 of RFC 2181) for its zone.
The OTBR DNS server responds RCode NotImp (4) on a query for the SOA record.

To Reproduce

setup: see #2683

In the dongle DNS client, send the query:

ot udp send fd90:5a2a:72b8:29f0:0:ff:fe00:fc33 53 -x 031d010000010000000000010764656661756c7407736572766963650461727061000006000100002905c0000000000000

Expected behavior

SOA record returned. RFC 1035 defines the fields.

This can be done directly by the OT stack code, which also answers "ipv4only.arpa" - no need to send the query to the host processor I think.

Console/log output
Can be added

Additional context
In cert testing, it was found some Thread devices perform the SOA query to verify liveness of the DNS server and verify presence of the default domain. The reference OTBR used in the test should be able to respond per IETF specifications.