[Transform] Can not group_by on field alias

[hendrikmuhs]

found by @wwang500

If group_by or certain aggregations use field aliases the mapping deduction does not resolve the alias to a concrete type but tries to create another alias in the destination index. As a result of wrong deduction the transform will fail on start when trying to create the destination index, see error below.

Workaround 1: Use the concrete fieldname instead of the alias
Workaround 2: Create the destination index manually

Solution: Deduction should use the proper type. The get mapping api does not support resolving the type, however field caps resolve the type correctly.

The wip PR for CCS support has a fix (so using field caps is the way to go for other reasons as well):

https://github.com/elastic/elasticsearch/pull/46075/files#diff-cbb75ff448d44acf01292ad7a635b22a

I suggest we cut out the mapping deduction change from this PR (@benwtrent)

Error:

{
  "error": {
    "root_cause": [
      {
        "type": "runtime_exception",
        "reason": "Could not create destination index [t2] for transform [t2]"
      }
    ],
    "type": "runtime_exception",
    "reason": "Could not create destination index [t2] for transform [t2]",
    "caused_by": {
      "type": "mapper_parsing_exception",
      "reason": "Failed to parse mapping [_doc]: The [path] property must be specified for field [hostname].",
      "caused_by": {
        "type": "mapper_parsing_exception",
        "reason": "The [path] property must be specified for field [hostname]."
      }
    }
  },
  "status": 500
}

[elasticmachine]

Pinging @elastic/ml-core