Override Geo aggregations in X-Pack

[nknize]

This is the second (of two) critical issues in preparation for geo_shape support for geo aggregations. To prevent a dependency on Value Source Refactor from further delaying the release of geo_shape doc values and aggregation support, a temporary solution of registering geo aggregations in the X-Pack spatial plugin is suggested by using the module framework. Much like the approach to overriding doc_values parameter, the SearchModule class will register geo_ aggregation implementations through a thin OSS geo module that provides the base classes necessary to implement Basic+ functionality in the X-Pack spatial module (when the appropriate license level is available). In this manner, the OSS implementations will only be registered absent a Basic+ license.

This approach allows the geo_shape doc value feature to move forward independent of any additional work to the aggregation framework paving the way to potentially release the feature earlier than anticipated.

[elasticmachine]

Pinging @elastic/es-analytics-geo (:Analytics/Geo)

[iverase]

@nknize Can this be closed?

[iverase]

I am closing this as it has been done and Elasticsearch 7.8 supports aggregations over geo_shape.