Support Deeply Nested Docs In Child Documents Transformer

As discussed in SOLR-12298, to make use of the meta-data fields in SOLR-12441, there needs to be a smarter child document transformer, which provides the ability to rebuild the original nested documents' structure.
In addition, I also propose the transformer will also have the ability to bring only some of the original hierarchy, to prevent unnecessary block join queries. e.g.

  {"a": "b", "c": [ {"e": "f"}, {"e": "g"} , {"h": "i"} ]} 

Incase my query is for all the children of "a:b", which contain the key "e" in them, the query will be broken in to two parts:
1. The parent query "a:b"
2. The child query "e:*".

If the only children flag is on, the transformer will return the following documents:

[ {"e": "f"}, {"e": "g"} ]

In case the flag was not turned on(perhaps the default state), the whole document hierarchy will be returned, containing only the matching children:

{"a": "b", "c": [ {"e": "f"}, {"e": "g"} ]