Refactors tailnet integration test and adds UDP echo tests with different MTU related to #15523
I still haven't gotten to the bottom of what's causing the issue (the added test case I expected to fail actually succeeds), but these integration test improvements are generally useful.
also:
* consolidates networking setup with easy and hard NAT
* consolidates client setup
* makes Client2 act like an agent at the tailnet layer, so it will send ReadyForHandshake and speed up the tunnel establishment
* adds support for logging tunneled packets
* adds support for dumping outer (underlay) IP traffic
* adds support for adjusting veth MTU
* adds support for IPv6 in the outer (underlay) network topology
re: #14715
This PR introduces the Coder service prefix: `fd60:627a:a42b::/48` and refactors our existing code as calling the Tailscale service prefix explicitly (rather than implicitly).
Removes the unused `Addresses` agent option. All clients today assume they can compute the Agent's IP address based on its UUID, so an agent started with a custom address would break things.
Spike Curtis