Make the path to `ffmpeg` customisable

[abey79]

What

Makes the path to ffmpeg customisable. Also hides the auto/soft/hard choice on native as it has not impact.

• Part of https://github.com/rerun-io/rerun/issues/8004

https://github.com/user-attachments/assets/4c76a3f1-407d-40ed-84a4-bdc20b25f22f

Checklist

• [x] I have read and agree to Contributor Guide and the Code of Conduct

• [x] I've included a screenshot or gif (if applicable)

• [x] I have tested the web demo (if applicable):

  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer

• [x] The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

• [x] If applicable, add a new check to the release checklist!

• [x] If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

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[Wumpf]

confirmed to work like a charm on Windows!