fix(firestore): doc and collection methods generic

[KingDarBoja]

Checklist

• Issue number for this PR: Feature Request: Make DocumentReference type generic #2504, Specify the return document type for AngularFirestoreDocument.get() #2361, AngularFirestoreCollection get function returns non generic QuerySnapshot #2438
• Docs included?: no
• Test units included?: no
• In a clean directory, yarn install, yarn test run successfully? yes

Description

Based on #2504 (comment), several methods for AngularFirestoreCollectionGroup, AngularFirestoreCollection and AngularFirestoreDocument now uses generics.

Take into account that not every method from the base API (firebase-js) support generics so I had to use the casting at some test specs.

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[KingDarBoja]

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[markgoho]

@KingDarBoja is there some documentation that we could also update as part of this PR?

[KingDarBoja]

Well there is no documentation regarding the use of generics on angularfire methods, so that's why I didn't make one for these changes.

[jamesdaniels]

Closing in favor of #2649, I was trying to solve merge conflicts with the web editor. Clearly it was not going well.