[FEATURE REQUEST] Query subcollection using structuredQuery

[ivanfemia]

Is your feature request related to a problem? Please describe.
I have a database structured as

collection1
|_document1
|_collection2
|_document2
|_fields

I want to query all documents in the collection2 that respond to a certain criteria.
Query method uses a url that can't be adjusted to this scenario as the actual URL should be projects/[PROJECT_ID]/databases/(default)/documents/collection1/document1:runQuery in stead of projects/[PROJECT_ID]/databases/(default)/documents/:runQuery

Describe the solution you'd like

• Add a parameter in the FirestoreQuery class to customize subpath var sub_collection_path: String = ""
• Change url path to include the subpath if set. var url: String = _base_url + _extended_url + query.sub_collection_path + _query_suffix

Describe alternatives you've considered
Tried different REST combination, but subcollection are not allowed in the where condition. In general '/' is not allowed

Additional context
This has been implemented and test with success in my project

[WolfgangSenff]

Hm. It used to be able to handle sub collections. I'm surprised it doesn't anymore. That said, if you have a fix, feel free to fork it and open a PR so I can take a look, happy to accept it.