Disable MapProvider variant when not available

[abey79]

What

This PR introduces a possibly over-engineered generic mechanism to control the availability of individual view property variant, and uses it to disable mapbox-based map background when the API key isn't properly set.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• 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
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!
• If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

• PR Build Summary
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[Wumpf]

I can see why you called this overengineered. The fact that I actually like it a lot might be quite telling about the state of things here 😅