Allow Blueprint types to be full Archetype/Components within a scoped…

… namespace (#4477)

### What
Blueprint code will benefit from being able to leverage the same
Archetype / Component patterns as our other datatypes.

Rather than treating Blueprint as an alternative ObjectKind this now
uses a `blueprint` scope for the given blueprint types. This had some
obnoxious fan-out to clean up all of the C++ header / include /
namespace / python import stuff.

Probably best reviewed commit by commit.

### 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 the web demo (if applicable):
  * Full build: [app.rerun.io](https://app.rerun.io/pr/4477/index.html)
* Partial build:
[app.rerun.io](https://app.rerun.io/pr/4477/index.html?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
- Useful for quick testing when changes do not affect examples in any
way
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG

- [PR Build Summary](https://build.rerun.io/pr/4477)
- [Docs
preview](https://rerun.io/preview/0e68b7923a60c2668be717a2638c626d5637cb15/docs)
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- [Examples
preview](https://rerun.io/preview/0e68b7923a60c2668be717a2638c626d5637cb15/examples)
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