Block embed: use dedicated attribute to save the embed ratio instead of using custom classes (#39616)

[petitphp]

What?

This PR change the way custom CSS classes for the aspect ratio are managed for the block embed.

Why?

When a block embed is transformed to a paragraph, the custom CSS classes for the aspect ratio were not removed.

How?

This is an alternative approach of #44947. This PR go a step further by adding a new attribute to store the aspect ratio and dynamically add the aspect ratio CSS classes to the block.

Testing Instructions

1. Open a post
2. Paste an embeddable link (eg. https://www.youtube.com/watch?v=S1Z9OHXME7E)
3. The field "Additional CSS Classes" in the "Advanced" panel should be empty
4. Publish the post
5. Go to the frontend
6. Inspect the block, the class attribute should contain the classes wp-embed-aspect-16-9 wp-has-aspect-ratio

[github-actions]

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Co-authored-by: petitphp <petitphp@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[Mamaduka]

There could be some overlap between this PR and the new aspect ratio controls for media block - #51138.