Enhancement Description

• One-line enhancement description (can be used as a release note): Add RunAsUserName field to set the user for a container's initial process on Windows
• Kubernetes Enhancement Proposal: https://github.com/kubernetes/enhancements/blob/master/keps/sig-windows/20190418-windows-security-context.md
  This covers both APIs that needed to be reviewed together, and it's implementable.
• Primary contact (assignee): @jsturtevant
• Responsible SIGs: SIG-Windows
• Enhancement target (which target equals to which milestone):
  • Alpha release target (x.y) 1.16
  • Beta release target (x.y) 1.17
    • doc pr Update RunAsUsername to beta website#17460
    • code pr Moving WindowsRunAsUserName to beta kubernetes#84882
  • Stable release target (x.y) 1.18
    • code pr Moving Windows RunAsUserName feature to GA kubernetes#87790
    • doc pr Removing beta references on RunAsUserName page website#19016

Existing issue:
kubernetes/kubernetes#73387

This existing PR will be updated and merged: kubernetes/kubernetes#73609

Past work in CRI layer:
kubernetes/kubernetes#64009