Clat eBPF program core implementation.

Test: atest netd_unit_test netd_integration_test
      installed on crosshatch-userdebug + manual testing
Bug: 65674744
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: If3a5de3647943c0ab97a1eb1d7a17c58308366e7