Add support for getting document's id in CollectionGroupQuery

[simonxabris]

Version info

Angular: 9.1.6

Firebase: 7.15.0

AngularFire: 6.02

Hi!

I'm using angularfire on a project and I've tried to use collectionGroup.valueChanges({ idField: 'docId' }) to retrieve a collection group with the document ids mapped to the docId property as I saw in the docs. When I tried this, TS complained that the valueChanges function does not expect any arguments, so I took a quick look at the source and found out that the implementation is missing.

I'll be providing a PR with the implementation of this shortly.

[jamesdaniels]

Addressed in 6.1 :)

[simonxabris]

Thank you for merging!