Add date format to DateHistogram source for Composite Aggregation

[russcam]

Relates: elastic/elasticsearch#28310

Add ability to specify a date format for DateHistogramCompositeAggregationSource. Property should be XML commented to indicate that it only applies to 6.3.0+

[Mpdreamz]

Started on this locally, still need to write an integration tests for it before submitting a PR though.

[russcam]

Closing as #3367 has been merged into 6.3