[Transform] HLRC builder ignores group_by order

[hendrikmuhs]

see https://github.com/elastic/elasticsearch/blob/master/client/rest-high-level/src/main/java/org/elasticsearch/client/transform/transforms/pivot/GroupConfig.java#L175

The group_by order can make a difference in terms of performance. In addition the order defined by the user should be preserved.

Note:

• HLRC gets replaced, I will open a separate issue for the new java client.
• transform internally re-orders group_by at runtime if it helps performance (since 7.15), however if 2 groups have the same order type, defined order is preserved

[elasticmachine]

Pinging @elastic/ml-core (Team:ML)

[hendrikmuhs]

After discussions we've decided to close this issue as no-fix. The reasons can be found in the other issue:

elastic/elasticsearch-java#99