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.

[qingyang-hu]

LGTM👍

[alcaeus]

Changes to the unified test format LGTM. Since we don't add operations to the schema, the unified test format does not get a version bump. Since this effectively prevents targeting a schema version in tests, please make sure to call out the requirement to implement getSnapshotTime in test runners in the ticket's downstream instructions.

[sanych-sun]

LGTM