Support GMSA for Windows workloads

@ddebroy

Enhancement Description

One-line enhancement description (can be used as a release note): Enable support for Group Managed Service Account (gMSA) for Windows workloads on Kubernetes.
Primary contact (assignee): @ddebroy
Responsible SIGs: sig-windows, sig-auth, sig-node
Design proposal link (community repo): https://github.com/kubernetes/enhancements/blob/master/keps/sig-windows/20181221-windows-group-managed-service-accounts-for-container-identity.md
and
https://github.com/kubernetes/enhancements/blob/master/keps/sig-windows/20190418-windows-security-context.md
Link to e2e and/or unit tests: Coming soon
Reviewer(s) - (for LGTM) recommend having 2+ reviewers (at least one from code-area OWNERS file) agreed to review. Reviewers from multiple companies preferred: @PatrickLang [Windows changes overall] @yujuhong [dockershim changes around Windows] @mikedanese, @liggitt [overall pod spec/container spec changes + authorization/security]
Approver (likely from SIG/area to which enhancement belongs): @PatrickLang @yujuhong @liggitt @mikedanese
Enhancement target (which target equals to which milestone):
- Alpha release target (1.14)
- Beta release target (1.16)
- Stable release target (1.18)

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