Implement CWL loop extension

[adamnovak]

CWL now has an extension that lets you loop!

https://cwltool.readthedocs.io/en/latest/loop.html

This is supposedly slated for CWL 1.3 as a core feature, so we should probably implement it now as an extension so we're ready. Toil is architected to let you do stuff like this, so hopefully it isn't too hard to implement.

┆Issue is synchronized with this Jira Story
┆Issue Number: TOIL-1679

[mr-c]

FYI: #4227 :-)