Include number of bytes to be scanned in IO EXPLAIN plan return value

[JamesRTaylor]

It'd be useful to include the estimated number of bytes to be scanned in the returned JSON value of an EXPLAIN (TYPE IO) command if statistics are available. Given the result is JSON, the return value could easily be parsed by client tools to give the user a rough idea of the time it will take to execute the query.

For example, a new estBytesScanned attribute could be added to the

{
  "inputTableColumnInfos": [
    {
      "table": {
        "catalog": "hive",
        "schemaTable": {
          "schema": "tpch",
          "table": "nation"
        }
      },
      "columns": [
        {
          "columnName": "regionkey",
          "type": "bigint",
          "domain": {
            "nullsAllowed": false,
            "ranges": [
              {
                "low": {
                  "value": "2",
                  "bound": "EXACTLY"
                },
                "high": {
                  "value": "2",
                  "bound": "EXACTLY"
                }
              }
            ]
          }
        }
      ],
    "estBytesScanned": "1230000"
    }
  ],
  "outputTable": {
    "catalog": "hive",
    "schemaTable": {
      "schema": "tpch",
      "table": "test_nation"
    }
  }
}