Improve background lazyload coverage for third party plugins

[abaicus]

All Submissions:

• Have you followed the Optimole Contributing guideline?
• Does your code follow the WordPress' 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:

Improves/implements third party background lazyload compatibilities for builder/block plugins, including:

• Kadence Blocks;
• Spectra Blocks;
• Essential Blocks;
• Elementor:
  • compatibility was deprecated because the main containers in Elementor were changed some time ago. We retain legacy compatibility and also fix the issue for newer versions;
  • moved the old Elementor selectors from the lazyload replacer class into the compatibility handler;
• Beaver Builder:
  • Here we were missing compatibility with their Box element, which acts like a container in the Beaver context. I believe this too was changed at some point;

Closes Codeinwp/optimole-service#1602.

How to test the changes in this Pull Request:

• Install the specified plugin.
• Make sure you have background lazyloading enabled in Optimole and have the Skip first x images option set to 0.
• For each builder/plugin:
  1. Create a new page that has elements with image backgrounds - sections/containers/rows would be the ones that are most probable to support backgrounds usually.
  2. Add a large spacer at the top (~2000px should be good enough) to make sure that the content goes out of screen.
  3. Visit the page in incognito - the backgrounds should be lazy-loaded.
  4. You can test that things are working by keeping the inspector open on the Network tab, filter by Img and reload the page - the images should not be loaded until you scroll down.

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?

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