CWL: implement the workflow-level loop extension

[mr-c]

During development of this, one would need to temporarily depend on common-workflow-language/cwltool#1641 until that is merged and a new release of cwltool is made

Example demonstration of finding the use of the loop extension (which will appear as an additional requirement on Workflow steps themselves)

diff --git a/src/toil/cwl/cwltoil.py b/src/toil/cwl/cwltoil.py
index 6d20984be..de63298ce 100755
--- a/src/toil/cwl/cwltoil.py
+++ b/src/toil/cwl/cwltoil.py
@@ -2689,6 +2689,8 @@ class CWLWorkflow(CWLNamedJob):
                         jobobj: Dict[
                             str, Union[ResolveSource, DefaultWithSource, StepValueFrom]
                         ] = {}
+                        loop = step.get_requirement("http://commonwl.org/cwltool#Loop")
+                        logger.info(f"Loop: {loop}.")
 
                         for inp in step.tool["inputs"]:
                             logger.debug("Takes input: %s", inp["id"])

Should we extend the toil-cwl Conditional object, or is another approach needed?

┆Issue is synchronized with this Jira Story
┆friendlyId: TOIL-1224