dynamic `DockerRequirement.dockerPull`

We have gotten this requirement often.

https://cwl.discourse.group/t/dynamic-changing-docker-pull-registry-location-in-dockerrequirements/530
https://cwl.discourse.group/t/using-javascript-in-requirements-such-as-dockerpull/814
(and many other times)

It would also improve the WDL to CWL converter, as WDL workflows often have one or more `string` inputs used to specify the name of a container for some particular step(s).

However some CWL workflow runners, like Arvados, want to know the names of all containers at workflow submission time (to copy them to shared storage)

@tetron suggests the addition of a `{ class: ContainerName, dockerPull: user_provided_value }` or similar. Then a workflow runner can scan the inputs object for `class: ContainerName` (and check the whole workflow DAG for any static `class: ContainerName`s) and handle the copying of those containers ahead of time.

Start with the two linked CWL Discourse discussions and the DockerRequirement.dockerPull behavior described in this issue. Trace how workflow inputs and the workflow DAG are represented, then define the specification change needed for runners to identify dynamic container names before submission.

Written by the indexing model from the issue text.