Firestore query-cursors/pagination: Added Query.startAfter (Android, iOS, js)

[tiagonuneslx]

Makes it possible to do basic pagination in Firestore.

Most basic example usage:

val firstPage = collectionReference.limit(10).get() // First 10 results
val lastDocumentSnapshot = firstPage.documents.lastOrNull() ?: return
val secondPage = collectionReference.startAfter(lastDocumentSnapshot).limit(10).get() // Second 10 results

Documentation:
https://firebase.google.com/docs/firestore/query-data/query-cursors

Example on how to do pagination using the Android native library:
https://medium.com/@ericampire/firestore-pagination-with-paging-3-762e371cc57f

Fixes part of #206

[nbransby]

@tiagonuneslx thank you for your contribution! could you add a test to for startAfter to this PR?

[tiagonuneslx]

@nbransby done!