mikedanese: add signer for istiod after 1.22

JobBuild
248
07-22 16:42
cloud-provider-gcp-e2e-create 1418253021216772096 1418250918184357888 1418250083954069504
cloud-provider-gcp-e2e-full 1418253021279686656 1418250918255661056 1418250084021178368
cloud-provider-gcp-tests 1418253021187411968 1418250918150803456 1418250083907932160
cloud-provider-gcp-verify-all 1418253021250326528 1418250918222106624 1418250083991818240