Dual-stack apiserver support

• One-line enhancement description (can be used as a release note): Make the apiserver service dual-stack in dual-stack clusters
• Kubernetes Enhancement Proposal: https://github.com/kubernetes/enhancements/tree/master/keps/sig-network/2438-dual-stack-apiserver
• Discussion Link: Discussed in sig-net on Jan 21, then a bit on slack (https://kubernetes.slack.com/archives/CGF3A900N/p1611328345018100, including a second thread off a later comment)
• Primary contact (assignee): @danwinship
• Responsible SIGs: sig-network, sig-apimachinery
• Enhancement target (which target equals to which milestone):
  • Alpha release target (x.y): ?
  • Beta release target (x.y): ?
  • Stable release target (x.y): ?
• Alpha
  • KEP (k/enhancements) update PR(s): KEP 2438: Dual-stack apiserver #2622, KEP-2438: dual-stack-apiserver updates / clarifications #2956
  • Code (k/k) update PR(s): Update apiserver endpoint reconcilers to support dual-stack kubernetes#117704
  • Docs (k/website) update PR(s): WIP dual-stack apiserver documentation website#32034

Please keep this description up to date. This will help the Enhancement Team to track the evolution of the enhancement efficiently.