SQL: support queries across indices with different mappings

[costin]

For ease of use, it should be possible to run SQL queries across multiple indices (which we support) with different mappings (which we do not).
This entails creating a virtual schema from merging the mapping and dealing with missing fields across the various indices.

The areas of investigation are dealing with the field source (especially for missing fields) and aggregations.

[elasticmachine]

Pinging @elastic/es-search-aggs

[bitkowitz]

This was the first major blocker that I encountered when trying to use SQL with the contents of my cluster.
Most of the indices in my cluster take advantage of elasticsearch's automatic mappings, so our log content
and schema can evolve overtime. It would be too onerous to enforce a common mapping across our daily log indices and we would lose one of the key benefits of elasticsearch for low-maintenance log ingestion.

[costin]

Hi,
It took a bit more time (with holidays and all) but this issue has been finally been resolved. Available in the upcoming 6.5.x and 7.x