ES|QL Support ST_DWITHIN

[craigtaverner]

Support for ST_DWITHIN was originally developed as part of the work on ST_DISTANCE for a few reasons:

• They are conceptually similar, and share some code
• PostGIS uses this function as the primary way to perform optimized index-backed searches by distance (PostGIS does not optimize ST_DISTANCE against the index, and recommends using ST_DWITHIN). Users coming from PostGIS will expect to find and use this function.
• We originally though we needed something like this for Lucene pushdown, such that we implement the pushdown of ST_DWITHIN to the existing _search query for geo_distance. Conceptually geo_distance is much closer to ST_DWITHIN than it is to ST_DISTANCE, because the distance value is strongly coupled to the function (as an argument).

However it was argued that we do not really need to implement ST_DWITHIN for a few reasons:

• We can optimize the pushdown to Lucene by rewriting ST_DISTANCE() < X to a call to ST_INTERSECTS(Circle(X)), which already supports lucene pushdown.
• Users can achieve all use cases with ST_DISTANCE itself, and do not really need a second function.

This PR captures the work done for ST_DWITHIN if we decide to implement it later.

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[elasticsearchmachine]

Hi @craigtaverner, I've created a changelog YAML for you.

[craigtaverner]

@elasticsearchmachine update branch