Add snapshot version to CheckpointInfo

[azliu0]

Describe your changes

This pr adds snapshot version to CheckpointInfo, in preparation for adding snapshot versioning to the worker.

Backward/forward compatibility checks

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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