* Add `Linux{Sandbox,Container}SecurityContext.SupplementalGroupsPolicy` and `ContainerStatus.user` in cri-api
* Add `PodSecurityContext.SupplementalGroupsPolicy`, `ContainerStatus.User` and its featuregate
* Implement DropDisabledPodFields for PodSecurityContext.SupplementalGroupsPolicy and ContainerStatus.User fields
* Implement kubelet so to wire between SecurityContext.SupplementalGroupsPolicy/ContainerStatus.User and cri-api in kubelet
* Clarify `SupplementalGroupsPolicy` is an OS depdendent field.
* Make `ContainerStatus.User` is initially attached user identity to the first process in the ContainerStatus
It is because, the process identity can be dynamic if the initially attached identity
has enough privilege calling setuid/setgid/setgroups syscalls in Linux.
* Rewording suggestion applied
* Add TODO comment for updating SupplementalGroupsPolicy default value in v1.34
* Added validations for SupplementalGroupsPolicy and ContainerUser
* No need featuregate check in validation when adding new field with no default value
* fix typo: identitiy -> identity
Kubernetes-commit: 552fd7e85084b4cbd3ae8e81ff13433e28dc8327
