Allow for `scan` with aggregations

In 5.0 elasticsearch allows a search request with aggregations when using scan/scroll which we should expose.

This has been moved here from elasticsearch-py - https://github.com/elastic/elasticsearch-py/issues/530

[macdjord]

To be clear - this is not about using a scrolled query to fetch the aggregations bit by bit. This is about having a query with both aggregations and hits, and you want to use a scrolled query for the hits while still seeing the aggregations.

Currently, there’s no way to do this: Search.execute() doesn’t do scrolled queries, while Search.scan() only return an iterator over the hits with no way to access the aggregation results.

My proposal is to add a new method, Search.execute_scan(), which returns a Response object like Search.execute(), but the Response.hits property, instead of being a static list, is a Search.scan()-style iterator.

[honzakral]

Just to clarify this is not scanning through results of aggregation, just returning an aggregates first and then scanning through the documents.

To “scan” over aggregations you can use the composite aggregation as shown here - https://github.com/elastic/elasticsearch-dsl-py/blob/master/examples/composite_agg.py