Pc min should match

Adds a `minimum_children` parameter to the `has_child` query and filter.  

* For the query with `score_mode: sum|avg|max`, it was possible to support this directly in ChildrenQuery.
* For the filter and the query with `score_mode: none`, I added CountChildrenConstantScoreQuery as it seemed a cleaner solution than trying to make either ChildrenQuery or ChildrenConstantScoreQuery fit

YAML tests have been added, but not sure what to do the Java tests.  Also still needs docs.

Closes #6019