Add new method "update_network" to "L3AgentExtension".

This new method will extend the L3 agent extensions functionality,
making them aware of any network change.

Change-Id: I7184c058ea59a1dceefda7ce4488df4d64c71111
Related-Bug: #1950454

neutron_lib/agent/l3_extension.py

@@ -58,3 +58,13 @@ class L3AgentExtension(extension.AgentExtension, metaclass=abc.ABCMeta):
         :param context: rpc context
         :param data: dict of router_id and new state
         """
+
+    @abc.abstractmethod
+    def update_network(self, context, data):
+        """Handle a network update event.
+
+        Called on network update.
+
+        :param context: RPC context.
+        :param data: dict of network data.
+        """

releasenotes/notes/l3-agent-extensions-update-network-e4887f7f258e40f0.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    A new method ``update_network`` has been added to ``L3AgentExtension``
+    to handle the network update events in the L3 agent extensions.