Fix json formatter for consistency with SQL

[dai-chen]

Issue #, if available:

Description of changes: The JSON response format was slightly different from SQL's in the datarows field (with one more layer row). This PR is to fix the inconsistency so existing client such as SQL CLI can handle both response in the same way.

Example with this fix:

~ curl -H 'Content-Type: application/json' -XPOST "http://localhost:9200/_opendistro/_ppl" -d'
{
   "query": "source=accounts | fields id"
}'
{
  "schema": [{
    "name": "id",
    "type": "integer"
  }],
  "total": 2,
  "datarows": [
    [4],
    [5]
  ],
  "size": 2
}

Previously without fix:

~ curl -H 'Content-Type: application/json' -XPOST "http://localhost:9200/_opendistro/_ppl" -d'
{
   "query": "source=accounts | fields id"
}'
{
  "schema": [{
    "name": "id",
    "type": "integer"
  }],
  "total": 2,
  "datarows": [
    {"row": [4]},
    {"row": [5]}
  ],
  "size": 2
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[chloe-zh]

LGTM, thanks for the fix!

[penghuo]

LGTM