Parallelize inventory rendering

[simu]

We add rayon as a dependency and adjust the inventory rendering logic to use par_iter() to render nodeinfos in parallel.

Additionally, the PR introduces a Python classmethod Reclass.set_thread_count() which allows users to control how many threads are spawned for the Rayon default threadpool. If that method is called mutliple times, any calls except the first one will do nothing and print a diagnostic message.

Checklist

• The PR has a meaningful title. The title will be used to auto generate the changelog
• PR contains a single logical change (to build a better changelog).
• Update the documentation.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency, internal
  as they show up in the changelog

[bastjan]

This is way too easy 😊 Would be cool to have a small benchmark linked somewhere.

[simu]

Would be cool to have a small benchmark linked somewhere.

I'll look into that in a follow-up