cluster-tools: refactoring follow-up

[jstriebel]

The following points are follow-ups from #858:

• [ ] Adapt map and map_to_futures signatures to fit the parent classes. This might include breaking changes.
• [ ] Add stricter typing & handling of Executor.__init__ and get_executor arguments.
• [ ] Add attrs classes that can be used for vx configs.
• [ ] Add missing functionality from vx to (new) abstract base class.
• [ ] Add wrapper for arbitrary executors (e.g. Dask distributed.Client, see this list, parsl executors, …)