feat(cover-block): add support for insert from url

[jenish77]

What?

Closes #10853

This PR adds "Insert from URL" support for the Cover block, allowing users to add image or video backgrounds by directly entering a URL instead of only being able to upload media or select from the Media Library.

Why?

Currently, the Cover block only allows users to add background media through uploading files or selecting from the Media Library. However, users often want to use images or videos hosted externally (e.g., CDN-hosted assets, stock photo URLs, etc.) without having to download and re-upload them.
Other media-related blocks like Image and Video already support "Insert from URL" functionality. This PR brings feature parity to the Cover block.

How?

1. Added onSelectURL prop to CoverPlaceholder component, which is passed to the underlying MediaPlaceholder component to enable the URL input UI.
2. Implemented getMediaTypeFromURL() async function in shared.js that detects whether a URL points to an image or video:
   • Fast path: First checks the file extension for common image/video formats
   • Content-Type detection: If extension detection fails, makes a HEAD request to check the Content-Type header
   • Fallback: If HEAD is not supported by the server, falls back to a GET request with Range: bytes=0-0 header
3. Added proper error handling:
   • If the media type cannot be determined (e.g., CORS restrictions, invalid URL, non-media Content-Type), a snackbar error notification is shown
   • The Cover block is NOT created with a blank background if detection fails

Testing Instructions

1. Open a post or page in the editor.
2. Insert a Cover block.
3. Click "Insert from URL" in the placeholder.
4. Enter a valid image URL and press Enter or click Apply.
5. Verify the image is loaded as the Cover background.

Screenshots or screencast

Before	After

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @jenish77! In case you missed it, we'd love to have you join us in our Slack community.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.