C++ codegen extensions, archetype tests, array ctors

[Wumpf]

What

Introduces a simple extension system for C++ codegen: Add an extra cpp file that will be compiled as part of the SDK. A section between two markes is copied into the generated hpp as part of generation (typically this section is removed from compilation via #ifdef)

Adds extensions to:

• color
• vec2/vec3/vec4
• quaternion
• origin3d
• point2d
• point3d
• arrow3d

... and uses them to simplify examples and test code!

All fully supported archetypes now have a simple test that checks that the base interface works and that we can serialize out to arrow without issues.

Additionally, there's tests for vecN/quaternion/color to check that their various constructors work as expected (not being a C++ expert it's fairly hard to predict)

Extends codegen with single-array-field-constructors.

---

• part of Code-gen for C++ #2647
• Fixes Support manual extensions to generated types #2798
• Fixes Arrow3D migration to archetypes #2785
• Missing only 2D example: Point{2|3}D migration to archetypes #2789
• Adds to Transform3D + DisconnectedTransform migration to archetypes #2791

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]

🤌