First PR to address #14244.
Adds common potential reasons as to why a direct connection to the workspace agent couldn't be established to `coder ping`:
- If the Coder deployment administrator has blocked direction connections (`CODER_BLOCK_DIRECT`).
- If the client has no STUN servers within it's DERP map.
- If the client or agent appears to be behind a hard NAT, as per Tailscale `netInfo.MappingVariesByDestIP`
Also adds a warning if the client or agent has a network interface below the 'safe' MTU for tailnet. This warning is always displayed at the end of a `coder ping`.
re: #13327
Adds local interfaces to `coder netcheck` and checks their MTUs for potential problems.
This is mostly relevant for end-user systems where VPNs are common. We _could_ also add it to coderd healthcheck, but until I see coderd connecting to workspaces over a VPN in the wild, I don't think its worth the UX effort.
Netcheck results get the following:
```
"interfaces": {
"error": null,
"severity": "ok",
"warnings": null,
"dismissed": false,
"interfaces": [
{
"name": "lo0",
"mtu": 16384,
"addresses": [
"127.0.0.1/8",
"::1/128",
"fe80::1/64"
]
},
{
"name": "en8",
"mtu": 1500,
"addresses": [
"192.168.50.217/24",
"fe80::c13:1a92:3fa5:dd7e/64"
]
}
]
}
```
_Technically_ not back compatible if anyone is parsing `coder netcheck` output as JSON, since the original output is now under `"derp"` in the output.
Currently, importing `codersdk` just to interact with the API requires
importing tailscale, which causes builds to fail unless manually using
our fork.
