[Firestore] add startAfter function to enhance pagination

Now retool only have startAt to support pagination in table components when using firestore resource.
According to Firestore official doc

Use the startAt() or startAfter() methods to define the start point for a query. The startAt() method includes the start point, while the startAfter() method excludes it.
For example, if you use startAt(A) in a query, it returns the entire alphabet. If you use startAfter(A) instead, it returns B-Z.

startAfter seems better than startAt in retool table components, so i suggest add startAfter function in retool