Cover: Enable background image controls

[im3dabasia]

What?

Closes #64703

Add Background Image panel support to the Cover block by enabling the backgroundImage support flag, matching the UI pattern used in the Group block.

Why?

Issue #64703 proposes adding a Background Image panel to the Cover block's Inspector, similar to the Group block's existing Background Image panel. Adds consistency among blocks.

How?

• Implemented as per the Group block's patterns.

Testing Instructions

1. Create or open a post/page
2. Insert a Cover block
3. Select the Cover block
4. Go to the Styles tab in the Inspector panel
5. You should see a Background image button
6. Click it to open the media picker and select or upload an image
7. The image should be applied to the cover block background
8. Verify focal point and other background controls are accessible

[github-actions]

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Co-authored-by: im3dabasia <im3dabasia1@git.wordpress.org>
Co-authored-by: richtabor <richtabor@git.wordpress.org>

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[youknowriad]

Thanks for this, @im3dabasia! #82751 added a Media section to the Cover block inspector using the same media control as Image and Media & Text, which resolved #64703. That approach replaces enabling the background image support here, so I'm closing this PR. If you think the Styles-tab placement is still worth pursuing, please open a new issue to discuss it.