CRI: Support Linux usernames for !linux platforms

The oci.WithUser option was being applied in container_create_linux.go
instead of the cross plat buildLinuxSpec method. There's been recent
work to try and make every spec option that can be applied on any platform
able to do so, and this falls under that. However, WithUser on linux platforms
relies on the containers SnapshotKey being filled out, which means the spec
option needs to be applied during container creation.

To make this a little more generic, I've created a new platformSpecOpts
method that handles any spec opts that rely on runtime state (rootfs mounted
for example) for some platforms, or just platform options that we still don't
have workarounds for to be able to specify them for other platforms
(apparmor, seccomp etc.) by internally calling the already existing
containerSpecOpts method.

Signed-off-by: Danny Canter <danny@dcantah.dev>

pkg/cri/sbserver/container_create_linux_test.go

@@ -31,6 +31,7 @@ import (
 	"github.com/containerd/containerd/contrib/seccomp"
 	"github.com/containerd/containerd/mount"
 	"github.com/containerd/containerd/oci"
+	"github.com/containerd/containerd/platforms"
 	imagespec "github.com/opencontainers/image-spec/specs-go/v1"
 	runtimespec "github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/opencontainers/selinux/go-selinux"
@@ -1506,7 +1507,7 @@ additional-group-for-root:x:22222:root
 			require.NoError(t, err)
 
 			spec.Root.Path = tempRootDir // simulating /etc/{passwd, group}
-			opts, err := c.containerSpecOpts(containerConfig, imageConfig)
+			opts, err := c.platformSpecOpts(platforms.DefaultSpec(), containerConfig, imageConfig)
 			require.NoError(t, err)
 			oci.ApplyOpts(ctx, nil, testContainer, spec, opts...)