Pass management URL to ClusterProxy (#289)

The proxy container defaults `--management-url` to
`https://api.netbird.io`, so on self-hosted NetBird the proxy talks to
SaaS and rejects the setup key as invalid.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added support for configuring a management URL for the cluster proxy
service.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

cmd/main.go

@@ -313,8 +313,9 @@ func main() {
 			os.Exit(1)
 		}
 		if err := (&controller.ClusterProxyReconciler{
-			Client: mgr.GetClient(),
-			ApiKey: netbirdAPIKey,
+			Client:        mgr.GetClient(),
+			ApiKey:        netbirdAPIKey,
+			ManagementURL: managementURL,
 		}).SetupWithManager(mgr); err != nil {
 			setupLog.Error(err, "Failed to create controller", "controller", "ClusterProxy")
 			os.Exit(1)