(Replaces  #2522)
Part of 
* #2649

For in-depth overview of how space views work now see #2533

Replaces custom user types for context and parts with typemap style
collections. These are filled every frame from a set of registered parts
& contexts. Registration happens on the `SpaceViewRegistry` where we
already register space view classes themselves.

Other rippling changes in overview:
* space view parts and context no longer have access to the space view's
state struct, this fully decouples them from any concrete space view
* space view part's data object is no longer strongly typed, but an
`&Any` instead, fullfills otherwise the same purpose
* `Scene`/`TypedScene` is gone now
* most of what was previously the scene buildup is now part of
`SpaceViewClass::ui`

For reviewing it's recommended to start with everything in the
`re_viewer_context` crate.


Overview of major trait entry points - see also #2533
<img width="1120" alt="image"
src="https://github.com/rerun-io/rerun/assets/1220815/ffdb1cdf-7efe-47a0-ac38-30262d770e69">




### What

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested [demo.rerun.io](https://demo.rerun.io/pr/2688) (if
applicable)

- [PR Build Summary](https://build.rerun.io/pr/2688)
- [Docs
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- [Examples
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