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/core/cmd/nanoswitch/BUILD b/core/cmd/nanoswitch/BUILD
new file mode 100644
index 0000000..c70e20f
--- /dev/null
+++ b/core/cmd/nanoswitch/BUILD
@@ -0,0 +1,40 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library")
+load("//core/build:def.bzl", "smalltown_initramfs")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["nanoswitch.go"],
+    importpath = "git.monogon.dev/source/nexantic.git/core/cmd/nanoswitch",
+    visibility = ["//visibility:private"],
+    deps = [
+        "//core/internal/common:go_default_library",
+        "//core/internal/common/supervisor:go_default_library",
+        "//core/internal/launch:go_default_library",
+        "//core/internal/network/dhcp:go_default_library",
+        "@com_github_google_nftables//:go_default_library",
+        "@com_github_google_nftables//expr:go_default_library",
+        "@com_github_insomniacslk_dhcp//dhcpv4:go_default_library",
+        "@com_github_insomniacslk_dhcp//dhcpv4/server4:go_default_library",
+        "@com_github_vishvananda_netlink//:go_default_library",
+        "@org_golang_x_sys//unix:go_default_library",
+        "@org_uber_go_zap//:go_default_library",
+    ],
+)
+
+go_binary(
+    name = "nanoswitch",
+    embed = [":go_default_library"],
+    pure = "on",
+    visibility = ["//visibility:public"],
+)
+
+smalltown_initramfs(
+    name = "initramfs",
+    files = {
+        ":nanoswitch": "/init",
+
+        # CA Certificate bundle
+        "@cacerts//file": "/etc/ssl/cert.pem",
+    },
+    visibility = ["//visibility:public"],
+)