feat(transport): nylon tunnels over layer 2/3

[encodeous]

Currently, nylon sends all traffic through a WireGuard tunnel.

It would be useful if nylon can send packets over specified interfaces without going through a tunnel. We can send over layer 2, and layer 3 (tun).

We can do this using gvisor's netstack.

This would allow:

• Higher performance for links over trusted networks (avoid encryption overhead)
• The use of other tunnels, like V2Ray, AmneziaWG, etc.
• (Advanced) subnet routing, we can send nylon IP packets directly over the physical interface.

Some implications I thought of so far:

• We would have to use raw sockets, so there would be new plumbing inside of polyamide
• We would need an alternative method of monitoring tunnel health (with polyamide, we have a custom header)
• Need to investigate netstack performance.