Add --threads / -j to control number of compute threads

[emilk]

What

• Closes CLI argument for setting max number of viewer threads #4931

You can now control the number of threads in the rayon thread pool using -j or --threads. The default is -2, meaning two less threads than the number of cores. This is to leave some breathing room for other threads the viewer spawns, as well as the rest of the users system.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
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• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
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[Wumpf]

lgtm!
please fix typo and fight me on clippy lint! 😄

[Wumpf]

I didn't know this method and I absolutely love the concept and its docs. For posterity: https://doc.rust-lang.org/std/thread/fn.available_parallelism.html