feat: add support for recursive queries

[craiglabenz]

Adds the ability for queries to begin fetching all nested documents regardless of their collection name. Previously, the only similar mechanic available was the all_descendants parameter (available via the collection_group class), which is imperfectly named and in fact only fetches all descendants with the same parent name. So, for example:

/users/alfred
/users/alfred/pets/scruffy
/users/alfred/users/alfreds-butler

In this case, running db.collection_group('users').get() would fetch /users/alfred and /users/alfred/users/alfreds-butler, but NOT /users/alfred/pets/scruffy.

Not that this is decidedly not all of Alfred's descendants.

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And thus: Recursive Queries ™️. Starting now, to fetch all three documents from the above hypothetical, a developer would run one of these two options:

db.collection_group('users').recursive().get()
# or this similar statement with an identical outcome:
db.collections('users').recursive().get()