feat(kubernetes): add DNS endpoint support to GKE cluster locator (#33297)

* feat(kubernetes): add DNS endpoint support to GKE cluster locator

Add endpointType config option to GkeClusterLocator allowing use of
DNS-based control plane endpoints instead of public IP endpoints.
When set to 'dns', the locator uses the cluster's DNS endpoint
(e.g. gke-<uid>.<region>.gke.goog) with fallback to public IP if
no DNS endpoint is available.

Signed-off-by: Rickard Dybeck <dybeck@spotify.com>

* fix: fix relevant AI review comments

Signed-off-by: Rickard Dybeck <dybeck@spotify.com>

---------

Signed-off-by: Rickard Dybeck <dybeck@spotify.com>

plugins/kubernetes-backend/config.d.ts

@@ -90,6 +90,13 @@ export interface Config {
           skipTLSVerify?: boolean;
           /** @visibility frontend */
           skipMetricsLookup?: boolean;
+          /**
+           * The type of endpoint to use for connecting to the cluster.
+           * 'public' uses the public IP endpoint (default).
+           * 'dns' uses the DNS-based control plane endpoint.
+           * @visibility frontend
+           */
+          endpointType?: 'public' | 'dns';
         }
     >;
     customResources?: Array<{