Implement EZP-21493: content fetch keyword filtered by subtree

As a developer, I would like to be able to fetch **keyword** and **keyword_count**, using the parent_node_id / depth parameter combination to limit the results.

Right now, the only limitation implemented is parent_node_id, which behaves as it does in a content/list function, with depth=1. (If parent_node_id is specified, only it's direct children will be considered, not it's children' children)

The goal is be to be able to use parent_node_id, with depth=0, to retrieve keywords and keyword_count from an whole subtree.