Add command to run a `coder speedtest` against a workspace.

`coder speedtest` tests the end-to-end connection speed of the tailnet tunnel to a workspace. Add this as an optional command to allow users to diagnose whether any perceived slowness is attributable to the networking.

It includes a JSON output format like:

```
Starting a 5s download test...
{
"overall": {
"start_time_seconds": 0,
"end_time_seconds": 5.0410079,
"throughput_mbits": 911.9123149955785
},
"intervals": [
{
"start_time_seconds": 0,
"end_time_seconds": 1.0028376,
"throughput_mbits": 803.0276966081047
},
{
"start_time_seconds": 1.0028376,
"end_time_seconds": 2.0064851,
"throughput_mbits": 819.0959315895271
},
{
"start_time_seconds": 2.0064851,
"end_time_seconds": 3.0181882,
"throughput_mbits": 961.8222263033492
},
{
"start_time_seconds": 3.0181882,
"end_time_seconds": 4.0363963,
"throughput_mbits": 988.6318523688822
},
{
"start_time_seconds": 4.0363963,
"end_time_seconds": 5.0410079,
"throughput_mbits": 985.311879735412
}
]
}
```

which lends itself to plotting or other nice graphical outputs.