support subfolder for s3 when sync with prefix set.

[xhuaustc]

When setting a bucket prefix, only sync the files under the folder, not files under the sub folders. This patch is to support syncing files under the sub folders.

[makseq]

@xhuaustc Hi! There is something wrong with your last commit (no differences with master), please check it.

[xhuaustc]

@xhuaustc Hi! There is something wrong with your last commit (no differences with master), please check it. @makseq Thanks. I checked, in the commit, , Delimiter='/' is deleted in Line 109. It's different from master.

Please check : https://github.com/heartexlabs/label-studio/pull/1042/files

[triklozoid]

@xhuaustc did you try "Recursive scan" storage option? With it storage should sync files under subfolders too.