Add nanoswitch and cluster testing

Adds nanoswitch and the `switched-multi2` launch target to launch two Smalltown instances on a switched
network and enroll them into a single cluster. Nanoswitch contains a Linux bridge and a minimal DHCP server
and connects to the two Smalltown instances over virtual Ethernet cables. Also moves out the DHCP client into
a package since nanoswitch needs it.

Test Plan:
Manually tested using `bazel run //:launch -- switched-multi2` and observing that the second VM
(whose serial port is mapped to stdout) prints that it is enrolled. Also validated by `bazel run //core/cmd/dbg -- kubectl get node -o wide` returning two ready nodes.

X-Origin-Diff: phab/D572
GitOrigin-RevId: 9f6e2b3d8268749dd81588205646ae3976ad14b3
diff --git a/BUILD b/BUILD
index 3d7283c..af96778 100644
--- a/BUILD
+++ b/BUILD
@@ -97,3 +97,9 @@
     name = "launch",
     actual = "//core/cmd/launch",
 )
+
+# Shortcut for launching a virtual network with two Smalltown instances
+alias(
+    name = "launch-multi2",
+    actual = "//core/cmd/launch-multi2",
+)