Beginnings of a solution to #12297
Doesn't cover disco or definitively display whether we successfully connected to DERP, but shows some checklist diagnostics for connecting to an agent.
For this first PR, I just added it to `coder ping` to see how we like it, but could be incorporated into `coder ssh` _et al._ after a timeout.
```
$ coder ping dogfood2
p2p connection established in 147ms
pong from dogfood2 p2p via 95.217.xxx.yyy:42631 in 147ms
pong from dogfood2 p2p via 95.217.xxx.yyy:42631 in 140ms
pong from dogfood2 p2p via 95.217.xxx.yyy:42631 in 140ms
✔ preferred DERP region 999 (Council Bluffs, Iowa)
✔ sent local data to Coder networking coodinator
✔ received remote agent data from Coder networking coordinator
preferred DERP 10013 (Europe Fly.io (Paris))
endpoints: 95.217.xxx.yyy:42631, 95.217.xxx.yyy:37576, 172.17.0.1:37576, 172.20.0.10:37576
✔ Wireguard handshake 11s ago
```
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