This adds support in tstun to utitilize the SelfNodeV4MasqAddrForThisPeer and
perform the necessary modifications to the packet as it passes through tstun.
Currently this only handles ICMP, UDP and TCP traffic.
Subnet routers and Exit Nodes are also unsupported.
Updates tailscale/corp#8020
Co-authored-by: Melanie Warrick <warrick@tailscale.com>
Signed-off-by: Maisem Ali <maisem@tailscale.com>