support read_time queries

[daniel-sanche]

The firestore API exposes a read_time option, which can be used to query the database at a specific point in time. We should expose this in the hand-written layers

[carlospero]

This read_time feature underpins PITR queries, yes? Highly useful there obviously.

I also recently read in the docs it can be used for more performant queries too:
https://firebase.google.com/docs/firestore/understand-reads-writes-scale#stale_reads

Adding support for this might be very appreciated by the community.

[carlospero]

While waiting for the release of this feature, I realized a question:

Do changes in this library somehow get incorporated into the firestore module in the firebase-admin python library?

Both can be used to query Cloud Firestore, but it will be an interesting situation if one library can make use of read_time while the other cannot.

Thanks!