Enable endpoint for cherrypicker plugin

Kubeflow community would like to [use cherrypicker Prow plugin](https://prow.k8s.io/command-help#cherrypick), similar to other Kubernetes projects: https://github.com/kubernetes/test-infra/blob/ab0aea453361484ad834f42c7cb633f5f74a3535/config/prow/plugins.yaml#L1826 According to the @cjwagner's comment currently this Prow instance doesn't have this service deployed: https://github.com/GoogleCloudPlatform/oss-test-infra/pull/2331#discussion_r1710130826 It would be nice to enable it. cc @zijianjoy @chensun @james-jwu @chaodaiG @chases2 @cjwagner @fejta @michelle192837 @mpherman2 @thesuperzapper