### What
* Closes https://github.com/rerun-io/rerun/issues/6909
* First steps on the road to
https://github.com/rerun-io/rerun/issues/3381
* Creates a new issue: https://github.com/rerun-io/rerun/issues/7117

Best reviewed ignoring whitespace changes.

The serializers for `Transform3D` and `LeafTransform3D` will now always
write every component, effectively clearing all previously logged
values.

`Transform3D` uses `Option<C>` for each component, treating it as a
single-element list component list where `None` is empty. If we ever
want to support updating some components using the transform (as
outlined in https://github.com/rerun-io/rerun/issues/3381) we would need
to turn this into a tristate, i.e. something like `NoneOrZeroOrOne<C>`.

`LeafTransform3D` uses `Option<Vec<C>>` for its components, but has no
interface yet for logging an update.

Both `Transform3D` and `LeafTransform3D` has a new `::clear()`
constructor in Rust, that replaced both `::new()` and `::default()`.
This is for added clarity, and to make the transition easier if we ever
add an `::update()` constructor, as per
https://github.com/rerun-io/rerun/issues/3381


![image](https://github.com/user-attachments/assets/588224b8-daec-463d-b238-8263f578ae19)

### 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):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7111?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7111?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!
* [x] If have noted any breaking changes to the log API in
`CHANGELOG.md` and the migration guide

- [PR Build Summary](https://build.rerun.io/pr/7111)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html

)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

---------

Co-authored-by: Andreas Reich <andreas@rerun.io>