Backport background image block support from Gutenberg

[andrewserong]

Backport background image block support feature that was introduced in Gutenberg in WordPress/gutenberg#53934.

Note this PR backports the PHP parts of the block support. This can be safely done prior to the JS packages landing for core, as the UI for the feature will not be exposed until the JS packages are updated.

Testing this PR manually in wordpress-develop is possible via the following steps:

• Update the Group block's block.json file to include background: { backgroundImage: true } under the supports key. I.e. that section will look like the following:

	"supports": {
		"__experimentalOnEnter": true,
		"__experimentalSettings": true,
		"align": [ "wide", "full" ],
		"anchor": true,
		"ariaLabel": true,
		"html": false,
		"background": {
			"backgroundImage": true
		},
...

• Skip the block.json cache so the changes to the block.json file are picked up. A quick way is to comment out the following lines:
  

  wordpress-develop/src/wp-includes/blocks.php

  Lines 363 to 365 in 7b90f22

  	if ( ! $core_blocks_meta ) {
  	$core_blocks_meta = require ABSPATH . WPINC . '/blocks/blocks-json.php';
  	}

• In a post or page, paste in the following block markup that displays a background image from wordpress.org:

<!-- wp:group {"style":{"background":{"backgroundImage":{"url":"https://i0.wp.com/wordpress.org/files/2022/10/community-photo-2-q50-unscaled.webp?w=2642","source":"file"}}},"layout":{"type":"constrained"}} -->
<div class="wp-block-group"><!-- wp:heading -->
<h2 class="wp-block-heading">A group block with a background image</h2>
<!-- /wp:heading --></div>
<!-- /wp:group -->

• Note: at this stage you won't yet see the background image rendered in the editor — this is to be expected as JS rendering will only happen once the JS packages are updated.
• Save the post or page and load the post on the site frontend. You should see that the background image is rendered:

Trac ticket: https://core.trac.wordpress.org/ticket/59357

---

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[mukeshpanchal27]

Thanks @andrewserong for PR. Some quick feedback.

[andrewserong]

Thanks for the quick code style fixes @mukeshpanchal27! 👍

[tellthemachines]

Changes LGTM! Thanks for adding tests ❤️

[tellthemachines]

Committed in r56614.