Automatic merge from submit-queue

CRI: add LinuxUser to LinuxContainerConfig

Following discussion in https://github.com/kubernetes/kubernetes/pull/25899#discussion_r70996068

The Container Runtime Interface should provide runtimes with User information to run the container process as (OCI being one of them).
This patch introduces a new field `user` into `LinuxContainerConfig` structure. The `user` field introduces also a new type structure `LinuxUser` which consists of `uid`, `gid` and `additional_gids`. 

The `LinuxUser` struct has been embedded into `LinuxContainerConfig` to leave space for future implementations which are not Linux-related (e.g. Windows may have a different representation of _Users_).

If you feel naming can be better we can probably move `LinuxUser` to `UnixUser` also. 

/cc @mrunalp @vishh @euank @yujuhong 
Signed-off-by: Antonio Murdaca <runcom@redhat.com>