Add a simple rust integration test with a mock cloud storage

[data-sync-user]

The official cloud storage library includes a mock: create a simple test simulating GCS writes.

Test case:

• GCS Offload payload successfully written, Spanner transaction successful/unsuccessful.

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[data-sync-user]

Taddes Korris commented from Jira:

Consider use of https://rust.testcontainers.org/