Add deeply nested documents URP

As discussed in SOLR-12298, there ought to be an URP to add metadata fields to childDocuments in order to allow a transformer to rebuild the original document hierarchy.

I propose we add the following fields:

1. _nestParent_
2. nestLevel
3. _nestPath_

_nestParent_: This field wild will store the document's parent docId, to be used for building the whole hierarchy, using a new document transformer, as suggested by Jan on the mailing list.

nestLevel: This field will store the level of the specified field in the document, using an int value. This field can be used for the parentFilter, eliminating the need to provide a parentFilter, which will be set by default as "level:queriedFieldLevel".

nestLevel: This field will contain the full path, separated by a specific reserved char e.g., '.'
for example: "first.second.third".
This will enable users to search for a specific path, or provide a regular expression to search for fields sharing the same name in different levels of the document, filtering using the level key if needed.