Better Facet Query Support

[joemcelroy]

Currently in v4.5, you are able to specify a custom facet query to override the default terms aggregation.

facet_attributes: [
{ 
  attribute: "type",
  field: "type.keyword",
  type: "string"
  facetQuery: (field, size, query) => ({ significant_terms: { field, size } })
}
]

This doesn't work well when the aggregation doesn't use the term bucket interface.

What we need is to add another function that transforms the elasticsearch agg response into the shape that Searchkit uses to present.