feat: OPNSense Topology useful to interact with only an opnsense instance.

With this work, no need to initialize a full HAClusterTopology to run
opnsense scores.

Also added an example showing how to use it and perform basic
operations.

Made a video out of it, might publish it at some point!

examples/ha_cluster/README.md

@@ -0,0 +1,15 @@
+## OPNSense demo
+
+Download the virtualbox snapshot from {{TODO URL}}
+
+Start the virtualbox image
+
+This virtualbox image is configured to use a bridge on the host's physical interface, make sure the bridge is up and the virtual machine can reach internet.
+
+Credentials are opnsense default (root/opnsense)
+
+Run the project with the correct ip address on the command line :
+
+```bash
+cargo run -p example-opnsense -- 192.168.5.229
+```