b2: Add b2-version-at flag to show file versions at time

Uses b2_list_file_versions to retrieve all file versions, and returns
the one that was active at the specified time

This is especially useful in combination with other backup tools, such
as restic, which may use rclone as a backend.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f