Give WorkGraph Task additional context about the ShellJob

[agoscinski]

In PR #164 the idea was to add support for linking inputs/outputs/arguments post-initialization of the WorkGraph task, but this is currently not possible because we need additional information what nodes are related to ShellJob which we don't have after initialization. This would require some more severe refactor as the ShellJob (and PythonJob) are merged to WorkGraph task on initialization. Since the benefits for the moment are only a nicer API and does not block any functionality, it is for now low-priority.