Make sure outbound UDP would use port-forward port instead of masquerade

[Frostman]

Basically if we have a GwPeer between VPC and internet with port forward 1000 from public IP to the server in VPC and masquerade to provide internet access to the VPCs, if server initiates UDP with source port 1000 we should just use 1000 as a source port and do not use masquerade.

[Frostman]

@qmonnet I'll try to confirm it asap, but side effects of having issues with tailscale points exactly to that.

[mvachhar]

So, the problem with tailscale etc. is not an issue with port forward, but the fact that we do not preserve IP and port for masquerade. There is a separate issue to check and handle, which is how we deal with UDP and port-forwards and what options we want.

[qmonnet]

Closing in favour of #1807, please re-open if necessary.