Skip transfers if file exists at destination #80
Description
Similar to #41 / #78 , and using the same XRD/S3 client calls, check for the presence of files at destination before transfer. This should be optional, and enabled via config like source checks. If a file is found, the transfer should be skipped rather than throwing an error. This means it would be possible for a request to result in no transfers, but still be valid and return no FTS job id. Ensure that this is handled gracefully.
Nice to have, not MVP.