feat: accessing task meta/requirements/hints from calling workflow

[a-frantz]

v1.2 of the specification introduced a way for tasks to access their own meta, requirements, and hints, but this is not easy to surface to workflows which may also have use of this information.

motivation: I have a WDL workflow which runs runs many tools and later funnels all those task outputs into a MultiQC task which generates a final report for users. MultiQC has a way to annotate those reports with the software version of each tool represented, but as of 1.2 there is no ergonomic way to supply that information from each called task to the final MultiQC task.

alternative: in v1.2, each tool could create an additional output with it's version number (either from task.meta.version or by parsing the task.container) and then the workflow could pass all those outputs into the MultiQC task, however this would add a lot of boilerplate to each task.

proposal: after task execution, there could be implied outputs added to every called task that would be accessed the same way as any other task output. For simplicity, this could just be the existing taskvariable that would persist after execution completes.

[patmagee]

I wonder if a few minor tweaks to the current would of the spec and making the task metadata available to the calling workflow would work?

1. What if we add another: container_sha, or tweak the wording of the spec so that the value there is not the tagged version of the container URI but the sha? ubuntue:latest -> `ubuntu@sha256:asdasd192d12d912md12d12
2. What if the task attribute was accessible from the call object

workflow hello_world {

   call say_greeting {
      ...
   }

 
  # say_greeting.task.container
  # say_greeting.task.cpu
  # say_greeting.task.memory
  # say_greeting.task.....
  
}