Change oci.WithUser behavior for LCOW
Change oci.WithUser behavior for LCOW

The `oci.WithUser` function relies on checking a path on the hosts disk to
grab/validate the uid:gid pair for the user string provided. For LCOW it's a
bit harder to confirm that the user actually exists on the host as a rootfs isn't
mounted on the host and shared into the guest, but rather the rootfs is constructed
entirely in the guest itself. To accomodate this, a spot to place the user string
provided by a client as-is is needed.

The `Username` field on the runtime spec is marked by Platform as only for Windows,
and in this case it *is* being set on a Windows host at least, but will be used as a
temporary holding spot until the guest can use the string to perform these same
operations to grab the uid:gid inside.

Signed-off-by: Daniel Canter <8d0fd95ae71f7e4870b010b10f83a6201b034a65@microsoft.com>

containerd/containerd,dmcgowan/containerd,estesp/containerd,estesp/containerd,thaJeztah/containerd,dmcgowan/containerd,thaJeztah/containerd,containerd/containerd,mikebrow/containerd,mikebrow/containerd