[FEATURE] Support Transform as an ISM action

[tanqiuliu]

Is your feature request related to a problem?
I would like to apply different kinds of aggregation on different subset of data from the source index as a part of the index lifecycle. Currently, transform supports dataSelectionQuery while rollup does not. Also, transform appears to support more aggregation types compared with rollup.

What solution would you like?
Support Transform as an ISM action. Similar to ISM rollup, the ISM transform should contain the following fields:

{
  "description": "Creating transform through ISM",
  "target_index": "target",
  "page_size": 1000,
  "data_selection_query": {
    "match_all": {}
  }
  "groups": [
    {
      "date_histogram": {
        "fixed_interval": "60m",
        "source_field": "order_date",
        "target_field": "order_date",
        "timezone": "America/Los_Angeles"
      }
    },
    {
      "terms": {
        "source_field": "customer_gender",
        "target_field": "customer_gender"
      }
    },
    {
      "terms": {
        "source_field": "day_of_week",
        "target_field": "day_of_week"
      }
    }
  ],
  "aggregations": [
    {
      "source_field": "taxless_total_price",
      "metrics": [
        {
          "sum": {}
        }
      ]
    },
    {
      "source_field": "total_quantity",
      "metrics": [
        {
          "avg": {}
        },
        {
          "max": {}
        }
      ]
    }
  ]
}

What alternatives have you considered?
Periodically monitor the lifecycle of index and keep creating transform jobs. But native support in ISM would be cleaner.

Do you have any additional context?
I can pick up the implementation if accepted

[tanqiuliu]

Created a PR #760