Support date histograms over date range fields

[jimmyjones2]

Describe the feature:

I've got a dataset that contains date range fields eg. employment start to finish dates. I'd like to plot a date histograms of the number of records it matched in a time period eg. how many employees did we have on each day of the last year

Elasticsearch version

6.4.1

[elasticmachine]

Pinging @elastic/es-search-aggs

[jpountz]

We agree that it would be a nice feature to support. Closing in favor of #23182 which already captures this need and has some discussion.