Feature: RuntimeSDK

[sbueringer]

• CAEPs:
  • https://github.com/kubernetes-sigs/cluster-api/blob/main/docs/proposals/20220221-runtime-SDK.md
  • https://github.com/kubernetes-sigs/cluster-api/blob/main/docs/proposals/20220414-runtime-hooks.md
• Feature gate state:
  • Alpha (disabled per default): v1.2
  • Beta (enabled per default): TBD
  • Stable (feature GA): TBD
  • Feature gate removed 1 release later: TBD