New response format to support visualization

[chloe-zh]

Is your feature request related to a problem? Please describe.
The observability visualization uses the SQL plugin as the query engine and consumes the PPL component of this plugin as the search language. The existing protocols are mostly to provide good service for the JDBC/ODBC connectors, but the frontend still needs to do much preprocessing before rendering the visualizations.

Describe the solution you'd like
The detailed proposal is noted in the comments of issue #162

Describe alternatives you've considered
Pending discussion for alternatives.

Additional context
See #162 for details.

[chloe-zh]

@mengweieric @ps48

Actually the format protocol is independent with the search/evaluation result, so the dimensions and axises should not be defined within the format. Instead, I suggest the frontend to gather necessary aggregation information when doing the drag&drop, for example to get the field to be aggregated on, and the name of the aggregated result etc.

The output protocol provides the data columns in the format that frontend can use directly, and the metat data of column names and types.

{
  "data": {
    "count()": [
      53,
      55,
      64,
      63,
      57,
      ...
    ],
    "timestime_span": [
      "2021-07-12 00:00:00",
      "2021-07-12 04:00:00",
      "2021-07-12 08:00:00",
      "2021-07-12 12:00:00",
      "2021-07-12 16:00:00",
      ...
    ]
  },
  "fields": [
    {
      "name": "count()",
      "type": "integer"
    },
    {
      "name": "timestime_span",
      "type": "timestamp"
    }
  ],
  "size": 252,
  "status": 200
}

[chloe-zh]

revised:

{
  "data": {
    "count()": [
      53,
      55,
      64,
      63,
      57,
      ...
    ],
    "timestime_span": [
      "2021-07-12 00:00:00",
      "2021-07-12 04:00:00",
      "2021-07-12 08:00:00",
      "2021-07-12 12:00:00",
      "2021-07-12 16:00:00",
      ...
    ]
  },
  "metadata": {
    "fields": [
      {
        "name": "count()",
        "type": "integer"
      },
      {
        "name": "timestime_span",
        "type": "timestamp"
      }
    ]
  },
  "size": 252,
  "status": 200
}

[dai-chen]

Closing and feel free to reopen if anything else needed. Thanks!