Add support for user namespaces

[rata]

Enhancement Description

• One-line enhancement description (can be used as a release note): Add support for user namespaces in pods
• Kubernetes Enhancement Proposal: https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/127-user-namespaces/README.md
• Discussion Link: This was discussed in several sig-node meetings, most notable Nov 2 2021 and in this PR, that feedback was incorporated already by this KEP is taking: keps/127: Support User Namespaces #2101.
• Primary contact (assignee): @rata @giuseppe
• Responsible SIGs: sig-node
• Enhancement target (which target equals to which milestone):
  • Alpha release target (x.y): 1.25
  • Beta release target (x.y):
  • Stable release target (x.y):
• Alpha
  • KEP (k/enhancements) update PR(s):
  • Code (k/k) update PR(s):
  • Docs (k/website) update PR(s):

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

[rata]

/sig node

[wojtek-t]

Closing - we're going to track that in #127