Feature request: S3 file access

Is S3 file access already supported? I could not find it mentioned in the documentation.

It was previously mentioned in this issue;

https://github.com/common-workflow-language/cwltool/issues/539

However that one was closed after `http` access was implemented. But as far as I can tell, this is not sufficient when you need to supply S3 access key and secret key to access the files.

As an example, Nextflow had this featured implemented, described here; https://www.nextflow.io/docs/latest/amazons3.html

so I was hoping to find an equivalent in `cwltool`

Thanks!