Add after_key to CompositeAggregation response

[russcam]

Relates: elastic/elasticsearch#28358
Documentation: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-composite-aggregation.html#_after

Add the after_key to the response of a Composite Aggregation. Including this will need to be done in backwards binary compatible fashion e.g. deriving a new type from MultiBucketAggregate<CompositeBucket>?

[russcam]

Closing this as #3367 has been merged into 6.3