clatd ebpf - implement egress ebpf program

ping/tcp to ipv4 literals over a cellular ipv6-only network continues
to work.

Unfortunately 'tcpdump -i v4-rmnet_dataX' no longer shows outgoing ipv4
tcp/udp/... traffic (only the icmp which is still handled by clatd)

Test: compiles, atest
Bug: 139396664
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I5e5106bd596f01232b3ca99649905238fd135ec7