onboarding: adding kubernetes to the project

[philips]

Goal: ensure all kubernetes releases can be verified and downloaded via rget.

Tasks:

• [ ] Add Kubernetes release automation for SHA256SUMS xref https://github.com/kubernetes/release/pull/849
• [ ] rget supports arbitrary URLs because of how
• [ ] Publish release sums for all old releases
• [ ] Create an auditor service (using crt.sh?) to email kubernetes-sig-release on changes against old releases, and ideally deduplicating against mirrors

cc @thockin @spiffxp @justinsb @justaugustus @dims