DRIVERS-2782: Expose snapshotTime

[papafe]

C# driver implementation POC: mongodb/mongo-csharp-driver#1777

TODO:

• Update changelog.
• Test changes in at least one language driver.
• Test these changes against all server versions and topologies (including standalone, replica set, and sharded
  clusters).
• Add unified tests.
• Add prose test for validation of input.
• Add information about the new getSnapshotTime operation (for unified tests).
• Add information about the new snapshotTime parameter of sessionOptions and that it must be retrieved from the entity map (for unified tests).
• Check that re-doing the query operation is best course of action for unified tests to verify that the snapshotTime does not get modified again.

[papafe]

TODO create ticket

[sanych-sun]

Let's create the ticket and remove this comment as non-informative.

[qingyang-hu]

LGTM👍

[sanych-sun]

Let's create the ticket and remove this comment as non-informative.

[sanych-sun]

Sorry for returning with this again, but I've just realized that we should mirror this changes into unified test format spec:
here: https://github.com/mongodb/specifications/blob/master/source/unified-test-format/unified-test-format.md#session-operations
and here: https://github.com/mongodb/specifications/blob/master/source/unified-test-format/unified-test-format.md#entity_session

[papafe]

I've mirrored this in the Session Operations section in unified test format spec. I did not modify the Entity Session section, as it already refers to the single spec documents, where I've already added the necessary info.