[nexus] add SRP-TC-1 for SRP registration and discovery (#12755)

This commit adds a new Nexus test case 1_3_SRP_TC_1 which verifies
SRP registration and discovery in a topology with a Border Router.

The test covers:
- SRP server registration in Thread Network Data.
- SRP service registration by a Thread End Device.
- Unicast DNS queries over the Thread interface.
- mDNS discovery over the Infrastructure interface.

Changes:
- Add tests/nexus/test_1_3_SRP_TC_1.cpp for test execution.
- Add tests/nexus/verify_1_3_SRP_TC_1.py for pcap verification.
- Update tests/nexus/CMakeLists.txt and run_nexus_tests.sh.
- Extend pktverify library (consts.py, layer_fields.py) to support
  DNS/mDNS and SRP-specific fields.

tests/scripts/thread-cert/pktverify/consts.py

@@ -351,6 +351,7 @@ WIRESHARK_OVERRIDE_PREFS = {
 }
 
 WIRESHARK_DECODE_AS_ENTRIES = {
+    'udp.port==53': 'dns',
     'udp.port==61631': 'coap',
     # SRP server ports for dissecting SRP requests and responses
     'udp.port==53535': 'dns',