[trel] small updates and adding trel test (#7205)

- Adds platform API `trelDnssdInitialize` to initialize TREL
  DNS-SD module.
- Adds `test_trel_connectivity.py` test.
- Fixed `Border Router` TREL test to really use TREL.

tests/scripts/thread-cert/node.py

@@ -125,6 +125,8 @@ class OtbrDocker:
             config.OTBR_DOCKER_IMAGE,
             '-B',
             config.BACKBONE_IFNAME,
+            '--trel-url',
+            f'trel://{config.BACKBONE_IFNAME}',
         ],
                                              stdin=subprocess.DEVNULL,
                                              stdout=sys.stdout,
@@ -1145,6 +1147,21 @@ class NodeImpl:
         self.send_command(cmd)
         return int(self._expect_result('\d+'))
 
+    #
+    # TREL utilities
+    #
+
+    def get_trel_state(self) -> Union[None, bool]:
+        states = [r'Disabled', r'Enabled']
+        self.send_command('trel')
+        try:
+            return self._expect_result(states) == 'Enabled'
+        except Exception as ex:
+            if 'InvalidCommand' in str(ex):
+                return None
+
+            raise
+
     def _encode_txt_entry(self, entry):
         """Encodes the TXT entry to the DNS-SD TXT record format as a HEX string.