[dsnsd-server] support handling of "A record" queries (#10364)

This commit adds support for responding to "A record" queries in the
DNS-SD server and discovery proxy.

If the query matches a host registered with the SRP server, the host's
IPv6 addresses are returned in the Additional Data section of the
response. If the query is resolved by the proxy, the `otPlatDnssd`
APIs are used to start/stop IPv4 address resolvers for the hostname
on the infrastructure network.

The `test_dnssd_discovery_proxy` unit test is updated to validate the
new functionality.

tests/scripts/thread-cert/border_router/test_dnssd_server.py

@@ -196,7 +196,7 @@ class TestDnssdServerOnBr(thread_cert.TestCase):
             })
 
         # check some invalid queries
-        for qtype in ['A', 'CNAME']:
+        for qtype in ['CNAME']:
             dig_result = digger.dns_dig(server_addr, host1_full_name, qtype)
             self._assert_dig_result_matches(dig_result, {
                 'status': 'NOTIMP',

tests/scripts/thread-cert/border_router/test_dnssd_server_multi_border_routers.py

@@ -278,7 +278,7 @@ class TestDnssdServerOnMultiBr(thread_cert.TestCase):
         self._verify_discovery_proxy_meshcop(br2_addr, br2.get_network_name(), host)
 
         # 4. Check some invalid queries
-        for qtype in ['A', 'CNAME']:
+        for qtype in ['CNAME']:
             dig_result = host.dns_dig(br2_addr, host1_full_name, qtype)
             self._assert_dig_result_matches(dig_result, {
                 'status': 'NOTIMP',