Add NetBird server to test connections (#328)

This change adds a NetBird server to the e2e test and ensures that
cluster proxy peers can start and connect to the server.

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

* **Changes**
* Updated the `ClusterProxy` `spec.apiServer` default to include a
trailing `/` when omitted, aligning operator behavior with the
Kubernetes in-cluster API URL.
* **Documentation**
* Refreshed the `ClusterProxySpec.apiServer` API reference to reflect
the trailing `/` default.
* **Tests**
* Improved end-to-end coverage with a dynamically provisioned management
URL, more robust readiness polling, and enhanced `ClusterProxy`
validation.
* **Chores**
* Updated the end-to-end test Go configuration and CI to use the
e2e-specific Go settings/toolchain.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

api/v1alpha1/clusterproxy_types.go

@@ -15,7 +15,7 @@ type ClusterProxySpec struct {
 
 	// APIServer is the URL of the Kubernetes API server to proxy requests to.
 	// +required
-	// +kubebuilder:default="https://kubernetes.default.svc.cluster.local"
+	// +kubebuilder:default="https://kubernetes.default.svc.cluster.local/"
 	APIServer string `json:"apiServer"`
 
 	// ServiceAccountName is a reference to the service account used for impersonation.