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

[spikecurtis]

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.