C++ as_components style & splatting

[Wumpf]

What

• Part of: C++ SDK V1 #2919
• Fixes Archetypes should implement as_components - C++ #3256
  • just like in Rust it's actually as_component_lists, we return a vector of type erased component arrays that know how to serialize themselves, dubbed AnonymousComponentList for now.
  • I'm not happy with naming and internals of AnonymousComponentList. This smells very much like we should have actual inheritance somewhere, but it's a bit tricky since we want to extract pointer out of arbitrary list types, giving rise to the type aware ComponentList. Overall the approach here tries to mimic Rust but ultimately misses the target, since, well this is not Rust. To be revisited, maybe it is just a naming issue!
• Fixes First-class citizenry for indicator components - C++ #3255
  • indicators are regular components now, their handling in as_component_lists is very slightly dubious since they're not backed by data but ofc indicator components don't have any data to begin with 🤷
• Fixes C++ component splatting support #3040
• First step towards C++ Component arrays (in Archetypes and log_components) need to be optional non-owning (& mappable) #3050

Trigger of this PR was the inability to deal with mono components like DrawOrder -> need splatting -> don't want to implement splatting separately for components & archetype -> implement as_components-style

Quick throw-away test for splatting radius on the random point demo:

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 demo.rerun.io (if applicable)

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[teh-cmc]

Looks great overall 👍 Just a handful of minor things.