[Transform] add support for extended_stats and string_stats aggregations

[hendrikmuhs]

#51808 added the groundwork for multi value aggregations in general and added percentiles support.

More multi value aggregations should be supported:

• stats
• extended stats
• string stats

[elasticmachine]

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

[hendrikmuhs]

Stats got added in #71850

[Kiriakos1998]

Hello @hendrikmuhs , is this issue still open so I may work on it? Also if it's still open do you suggest opening two PRs for each aggregation or one for both of them?

[hendrikmuhs]

@Kiriakos1998 Either way, 1 combined or 2 separate PR's, works. Have a look at #71850 to see how it works, it should be relatively easy to add support for both. The main work is writing good tests.