cmd/k8s-operator: Allow custom ingress class names (#16472)

This commit modifies the k8s operator to allow for customisation of the ingress class name
via a new `OPERATOR_INGRESS_CLASS_NAME` environment variable. For backwards compatibility,
this defaults to `tailscale`.

When using helm, a new `ingress.name` value is provided that will set this environment variable
and modify the name of the deployed `IngressClass` resource.

Fixes https://github.com/tailscale/tailscale/issues/16248

Signed-off-by: David Bond <davidsbond93@gmail.com>

cmd/k8s-operator/deploy/manifests/operator.yaml

@@ -5129,6 +5129,8 @@ spec:
                             fieldPath: metadata.namespace
                     - name: OPERATOR_LOGIN_SERVER
                       value: null
+                    - name: OPERATOR_INGRESS_CLASS_NAME
+                      value: tailscale
                     - name: CLIENT_ID_FILE
                       value: /oauth/client_id
                     - name: CLIENT_SECRET_FILE