[filters] filter operator registry and spatial_filter operator

In Maps, users can create spatial filters. The filter is added to the Filter bar.

There are 2 important properties about spatial filters created from a Map:

filter.meta.type is set to spatial_filter. Maps uses filter.meta.type to identify spatial filters. Spatial filters are displayed on the map to provide a visual reference of all applied spatial filters.
filter.meta.isMultiIndex is set to true since Elasticsearch Query DSL for the filter is not tied to a single index pattern or field. The Elasticsearch Query DSL is generated in such a way that the filter works for all map layers, regardless of index pattern or spatial field. In the screenshot above a single filter is filtering both layers, even though they are from different index patterns and different spatial fields.
- Each layer identifies one or more geo_point or geo_shape fields that is used to display features for that layer
- The Elasticsearch Query DSL combines these into a bool.should clause where each field provides a bool.must clause ensuring the spatial field exists and that the document satisfies the spatial filter. Any document matching any of the bool.must clauses allows the bool.should clause to produce a match.

elastic / kibana