[Feature Request] Allow custom min_doc_count in queries

# Summary

We often make queries like 'give me all sales for this specific month grouped by day'. When we do this, it would be helpful to still return a node even when there are no documents. Providing these empty nodes is still helpful for our clients, who maybe want to still display a point in a graph with a 0 value.

It looks like the [`min_doc_count` is hard coded as 1](https://github.com/block/elasticgraph/blame/78715bcaf7463b3ae126f8cc7e08e5184aea4306/elasticgraph-graphql/lib/elastic_graph/graphql/aggregation/date_histogram_grouping.rb#L45-L49) to intentionally not return empty buckets, but having the ability to override this and return empty buckets would be very helpful!