[dashboard] Add support for log, apm and process query in legacy API

[jbenais]

This PR adds support for log_query, apm_query and process_query in legacy screenboards and timeboards API.

Datadog JSON sample

{
  "requests": [
    {
      "log_query": {
        "index": "main",
        "search": {
          "query": "service:my-app AND status:error"
        },
        "groupBy": [],
        "compute": {
          "aggregation": "count",
          "facet": null,
          "interval": "300000"
        }
      },
      "aggregator": "count",
      "style": {
        "palette": "warm",
        "type": "solid",
        "width": "thick"
      },
      "type": "line",
      "conditional_formats": []
    }
  ],
  "viz": "timeseries",
  "autoscale": true,
  "yaxis": {
    "scale": "log",
    "includeZero": false
  },
  "status": "done"
}

[enbashi]

Nice work!

[bkabrda]

The code is clean and the tests are passing. I have no objections here. Good work 👍 , I'm merging this PR.