Introduce new nest path query parser

Since the advent of the special field  nest_path (for nested documents), the [child childFilter=...] query supports a barely documented syntax for matching children based on the path in this field.  It's currently implemented here: `org.apache.solr.response.transform.ChildDocTransformerFactory#processPathHierarchyQueryString`.  It occurred to me that this would be better formed as a query parser so that it might be used in other contexts.  Furthermore the block join query parsers could detect and use it just as the child doc transformer does so today. This is just an idea; I haven't fully thought it through.

One small change I'd suggest to the current detection logic is that if the query starts with a '{' then don't try to guess if it's a path style query.