clat: add support for ESP, mirroring existing GRE support

This relies on some pretty heavy duty magic on the side of the PLAT
nat gateway in order to be useful.

It might not even work... but it already doesn't work, so it can't
be worse then the current state where we simply discard these packets.

Test: builds
Bug: 139499742
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I3adfae6251c66125ff31c23c7967feacafb805a0
3 files changed