[WIP] Add support for non-backslash delimiters

[ZhuTopher]

What changes are proposed in this pull request?

This PR changes the behaviour of the S3 REST API to no-longer throw "Alluxio S3 API only support / as delimiter." when calling ListObjects(V2) with delimiter set to values other than /.

• For context, see this comment thread on this PR: https://github.com/Alluxio/alluxio/pull/15333#discussion_r854621242

Also, there is a new property key alluxio.proxy.s3.list.objects.return.folders to control how to return folders in the response of ListBucketResult:

ListObjects(V2) has disparity on how to handle “folders” created in S3:

• Folders created via AWS S3 web UI are included in response with size 0
  • We are currently using this implementation (i.e: alluxio.proxy.s3.list.objects.return.folders=true by default
• “Logical” folders created as parent paths to objects (i.e: dir/ from PUT bucket/dir/object) are not included

Why are the changes needed?

Improves conformity with the S3 API.

Does this PR introduce any user facing changes?

New property key alluxio.proxy.s3.list.objects.return.folders.