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!

opnsense-config-xml/src/data/opnsense.rs

@@ -216,7 +216,7 @@ pub struct System {
     pub maximumfrags: Option<MaybeString>,
     pub aliasesresolveinterval: Option<MaybeString>,
     pub maximumtableentries: Option<MaybeString>,
-    pub language: String,
+    pub language: Option<MaybeString>,
     pub dnsserver: Option<MaybeString>,
     pub dns1gw: Option<String>,
     pub dns2gw: Option<String>,