Firestore getDocument should have optional parameter as source(Cache, Server or Default)

[kshivang]

This could be useful in scenario like implementing custom get strategy like, check in Cache first then check on server, which is opposite of default get method of firebase Firestore. Though they have optional parameter called source in there, documentRef.get(source: Source = Source.DEFAULT).

Same thing could be implemented with getDocument method of RxFirestore