[posix] update Posix::Resolver to support RDNSS-discovered recursive DNS servers (#11342)

Key Changes:
* Server Prioritization: RDNSS-discovered servers are prioritized
  based on their advertised lifetime. Servers with longer lifetimes
  are preferred.
* Recursive DNS Server List: The resolver maintains a list of
  recursive DNS servers, which now includes servers learned via RDNSS.
* DNS Query Integration: The resolver uses the RDNSS-learned servers
  when forwarding DNS queries upstream.
* Border Routing Integration: The RDNSS callback is registered to the
  border routing module.

Minor Changes:
* The unused function `Transaction *GetTransaction(int aFd)` is
  removed.

tests/scripts/thread-cert/node.py

@@ -4145,6 +4145,28 @@ EOF
 """ % (prefix,))
         self._start_radvd_and_verify()
 
+    def start_rdnss_radvd_service(self, dns_server_address):
+        self.bash(f"""cat >/etc/radvd.conf <<EOF
+interface eth0
+{{
+    AdvSendAdvert on;
+
+    AdvReachableTime 20;
+    AdvRetransTimer 20;
+    AdvDefaultLifetime 180;
+    MinRtrAdvInterval 120;
+    MaxRtrAdvInterval 180;
+    AdvDefaultPreference low;
+
+    RDNSS {dns_server_address}
+    {{
+        AdvRDNSSLifetime 1800;
+    }};
+}};
+EOF
+""")
+        self._start_radvd_and_verify()
+
     def stop_radvd_service(self):
         self.bash('service radvd stop')