[Feature]: Subworkflows (Parallelization of steps)

[hanstrompert]

Background

This issue might be broken down in smaller pieces of work. In terms of parallelization there are 2 main usecases:

• when certain tasks in one workflow are suited to be executed in parallel rather then sequentially: for example gathering info from multiple independent sources.
• when a workflow act as a parent workflow that spawns children workflows: for example when you want to migrate in parallel all the subscriptions referring to a port as a consequence of migrating that port. -> #783

Bonus idea: it might be worth when evaluating this story to consider possible strategies to implement a choice point. The use-case is that in certain cases you might want to finish a workflow based on some external conditions. Currently the only option is to abort it.

Implementation