[ML-DataFrame] remove array arguments for group_by

[hendrikmuhs]

remove array arguments for group_by

This simplifies the syntax from

"pivot": {
      "group_by": [
        {
          "reviewer": { "terms": { "field": "reviewer_id" } }
        },
...
      ],
      "aggs": {
          "avg_rating": { "avg": { "field": "rating" } }
        },
...
  }

to

"pivot": {
      "group_by": {
          "reviewer": { "terms": { "field": "reviewer_id" } }
        },
...
      "aggs": 
        {
          "avg_rating": { "avg": { "field": "rating" } }
        },
...
  }

The extra square brackets were not necessary as the inner groupings are uniquely named.

[elasticmachine]

Pinging @elastic/ml-core