Add nested aggregation support in transform

[benwtrent]

Nested aggs allow unique interactions to nested fields stored in source indices. For certain interactions, these are absolutely required.

The implementation might prove difficult.

• Since the contained aggs in the nested definition are "just aggs", the current parsing + mapping logic can be used
• considerations will have to be made around destination mapping type + destination field names. How does the nested path play into this?

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)

[tom-x1]

Any progress here?

[realsuayip]

Has anyone been able to achieve this functionality with some script magic? I'm trying to normalize nested field items to another index as documents.