Support other bucketing aggregations

It would be amazing if this plugin supported other bucketing aggregations such as Significant Terms for a filtered expression.
Right now I can only use a Terms aggregation, which is only really useful when I plan to plot all documents (of a certain type) in my collection.

Start by locating where the plugin currently configures and renders Terms aggregations for filtered expressions. Compare that flow with Elasticsearch's Significant Terms aggregation requirements and identify the relevant integration points. Done means the plugin supports the requested bucketing aggregation and can use it in a filtered expression.

Written by the indexing model from the issue text.