Support execution_hint field in composite aggregation sources

[kylelyk]

Description

Normal Terms Aggregations supports supplying the execution_hint field to avoid using global ordinals to filter on high-cardinality fields. However, this feature is not supported in the list of sources for Composite Aggregations, which means that high-cardinality fields (like a UUID) cannot be bucketed on efficiently even if a filtering query is run first. I would like execution_hint to be added so that bucketing by high-cardinality fields is possible for composite aggregations by setting it to "map". It seems that Kibana's autocomplete functionality already thinks that field is supported which is a bit confusing as well.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)