Implement DHCPv4 default callbacks

This implements common callbacks to manage interface IPs and
routes in the kernel from DHCPv4.

Test Plan: New integration tests against our kernel via ktest.

X-Origin-Diff: phab/D657
GitOrigin-RevId: 3c39dddbd0e4151e6e902de150243296e6e459b4
diff --git a/third_party/linux/linux-smalltown.config b/third_party/linux/linux-smalltown.config
index a608bbd..df38412 100644
--- a/third_party/linux/linux-smalltown.config
+++ b/third_party/linux/linux-smalltown.config
@@ -1483,7 +1483,7 @@
 CONFIG_NETDEVICES=y
 CONFIG_NET_CORE=y
 # CONFIG_BONDING is not set
-# CONFIG_DUMMY is not set
+CONFIG_DUMMY=y
 CONFIG_WIREGUARD=y
 # CONFIG_WIREGUARD_DEBUG is not set
 # CONFIG_EQUALIZER is not set