Async search to expose partial search hits

[javanna]

The async search API allows to retrieve partial aggregations results. We would like to also expose partial search hits.

That means allowing users to run sorted queries and aggregations in the same search request efficiently while today the recommendation is to split the request to retrieve top documents independently of aggregations.

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[javanna]

This has been open for quite a while with no activity. While it is something that may be interesting to solve, it does not align with our current priorities. For now I'm going to close this as something we aren't planning on implementing. We can re-open it later if needed.

Note that cross-cluster search does expose partial top hits when using minimize roundtrips, meaning whenever each remote cluster is done with its own part of the search, its results are directly exposed to the async search API.