Fix: add missing Carousel block attribute

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

#746 added a new attribute to the Carousel block, but it was not registered on the backend - this fixes that.

Ideally the block attributes should be defined in a single JSON file, so that's the next step.

How to test the changes in this Pull Request:

1. On master, insert a Carousel block
2. Observe a PHP error logged about missing 'imageFit' index
3. Switch to this branch, observe that all is well now

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[matticbot]

🎉 This PR is included in version 1.25.1 🎉

The release is available on GitHub release

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