profiling support

[jdx]

there should be a way to get information about execution times with hk, something like this json (wall vs cpu for parallelization time):

{
  "wall_ms": 10000,
  "cpu_ms": 20000,
  "job_count": 2,
  "steps": {
    "prettier": {
      "wall_ms": 10000,
      "cpu_ms": 20000,
      "job_count": 2
    }
  }
}

this could be output via the CLI with hk run pre-commit --profile or hk run pre-commit --report. We'd also need to be able to use it inside of a config with events:

events {
  ["report"] = "echo $HK_PROFILE # contains json"
}

or maybe just as a separate step using a tera template:

hooks {
  ["pre-commit"] {
    ["fix"] = new Fix {}
    ["report"] {
      exclusive = true
      env {
        ["HK_REPORT"] = "{{hk_report}}"
      }
      run = "echo $HK_REPORT # contains json"
    }
  }
}