Add a attr.rust.no_placeholder attribute to codegen

[abey79]

What

This attribute sets the placeholder field of the component's reflection to None instead of Some(C::default()). This makes it possible for the component not to implement Default, but make it incompatible with the fallback system (this would trigger the exisiting ComponentFallbackError::MissingPlaceholderValue error).

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

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[Wumpf]

Got #7447 far enough to be sure that that replaces it!