Simplify RRM JS CTA type registration

## Feature Description

Adding a Reader Revenue Manager CTA type on the datastore currently means touching four compile-enforced maps in `assets/js/modules/reader-revenue-manager/datastore/cta-types/`:

- `CTA_TYPES`
- `CallToActionConfigByType`
- `CallToActionByType`
- `CTA_TYPE_HANDLERS`

That was an intentional choice for [#13006](https://github.com/google/site-kit-wp/issues/13006) / [#13208](https://github.com/google/site-kit-wp/pull/13208): missing one of those spots is a `tsc` error on that line. PHP already registers handlers in one list and keys them off `get_type()`.

With a second CTA type, keeping four maps in sync is more costly. This ticket simplifies that to a small number of/one registration point without changing create/list behavior.

See relevant discussions:
1. https://github.com/google/site-kit-wp/pull/13208#discussion_r3810194883
2. https://github.com/google/site-kit-wp/pull/13208#issuecomment-5344974178
3. https://github.com/google/site-kit-wp/pull/13208#pullrequestreview-5000780779

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## Acceptance criteria

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## Implementation Brief

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### Test Coverage

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## QA Brief

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## Changelog entry

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