Track source for arrays of objects

[kkrik-es]

This PR uses option store_array_source for objects to track the source of (sub)object arrays in synthetic mode. This allows preserving the original source, as synthetic mode merges, deduplicates and sorts array entries by default. The downside is that the whole object array (including children objects and fields) gets stored twice, while there's additional overhead while synthesizing source at query time.

This functionality will be experimental initially, to get some experience before documenting and opening up its use.

Fixes #90708

[elasticsearchmachine]

Hi @kkrik-es, I've created a changelog YAML for you.

[kkrik-es]

Should we add documentation for new parameter?

I was thinking of keeping it experimental initially, until Observability provides feedback on its use. @felixbarny fyi.

[felixbarny]

The store_array_source flag looks good to me, I've pinged the team to test it and provide feedback.