Add containerd & gVisor support

This adds containerd, CNI, gVisor and all the necessary shims
and supporting infrastructure. It also enables all relevant features in
the Linux kernel. containerd is designed as a simple supervisor.Runnable.
It is not being started yet, this will happen in D497.

Split out from feature/kubelet.

Test Plan:
Has been tested in conjunction with the rest of D497, will be
covered by a K8s E2E test there.

X-Origin-Diff: phab/D509
GitOrigin-RevId: 92523516b7e361a30da330eb187787e6045bfd17
diff --git a/core/internal/containerd/loopback.json b/core/internal/containerd/loopback.json
new file mode 100644
index 0000000..f375c5d
--- /dev/null
+++ b/core/internal/containerd/loopback.json
@@ -0,0 +1,4 @@
+{
+  "cniVersion": "0.3.0",
+  "type": "loopback"
+}