Depends on D4083

Reading log id from the client. Log id is optional, if there is no log id provided, then it means we're appending an attachment to the backup. If the log id is present, then we're appending it to the log item. In this case, we also check whether a log item with the given log id exists in the database.

The backup service can be built successfully.

For the full test plan of this functionality that has been broken into multiple diffs, please see D4090.