Add dynamic work rebalancing to Firestore Query Source

[damccorm]

Today, Firestore does not currently have API support for splitting a query dynamically and as such does not allow the Query Source to be implemented as a Splittable DoFn. Instead a query is split ahead of time using Partition Query.

Imported from Jira BEAM-12605. Original Jira may contain additional context.
Reported by: benwhitehead@google.com.