cmd/k8s-operator,k8s-operator: allow using LE staging endpoint for Ingress
Allow to optionally use LetsEncrypt staging endpoint to issue
certs for Ingress/HA Ingress, so that it is easier to
experiment with initial Ingress setup without hiting rate limits.
Updates tailscale/corp#24795
Signed-off-by: Irbe Krumina <irbe@tailscale.com>