[posix] handle dynamic infra if status (#6186)

This commit includes below changes:

- handles dynamic infrastructure interface status changes. The Routing
  Manager will be started if the infra interface status turns to be
  RUNNING and a valid link-local address is present. Otherwise, the
  Routing Manager will be stopped.

- changes the default status of the Routing Manager from enabled to
  disabled and it is initially enabled for the posix platform.

- the posix implementation now doesn't require the presence of the
  link-local address at initialization stage. In this case, the
  Routing Manager will not be started before a link-local address is
  added to the infra interface.

- add CI tests for the new behavior.

tests/scripts/thread-cert/node.py

@@ -2592,6 +2592,18 @@ class LinuxHost():
     PING_RESPONSE_PATTERN = re.compile(r'\d+ bytes from .*:.*')
     ETH_DEV = config.BACKBONE_IFNAME
 
+    def enable_ether(self):
+        """Enable the ethernet interface.
+        """
+
+        self.bash(f'ifconfig {self.ETH_DEV} up')
+
+    def disable_ether(self):
+        """Disable the ethernet interface.
+        """
+
+        self.bash(f'ifconfig {self.ETH_DEV} down')
+
     def get_ether_addrs(self):
         output = self.bash(f'ip -6 addr list dev {self.ETH_DEV}')