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.
Adds checks to coderd/healthcheck/derphealth for STUN issues:
- Alerts if there is not least one healthy STUN server,
- Alerts if we see variable port mapping.
Relates to #8965
- Added error codes for separate code paths in health checks
- Prefixed errors and warnings with error code prefixes
- Added a docs page with details on each code, cause and solution
Co-authored-by: Muhammad Atif Ali <atif@coder.com>
