fix: compatibility with otter blocks background images defined in CSS files

[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:

• Fixes issue where CSS files generated on post save didn't have the image URLs replaced;
• Properly sets up the replacer on the specific rest routes used to generate the CSS files;
• This was previously already working when regenerating the CSS, but not when saving the post;

Closes Codeinwp/optimole-service#1594.

How to test the changes in this Pull Request:

1. Install Otter Blocks;
2. Add the following line in a .php file under /wp-content/mu-plugins to make sure CSS is generated regardless of how large the Otter styles on a page are:

• add_filter('styles_inline_size_limit', '__return_zero');

3. Create a page containing one or more Otter section blocks and add background images to them;
4. Save and publish the page;
5. On the frontend, check the browser inspector, the background image should be loaded from Optimole CDN;

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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🎉 This PR is included in version 4.2.0 🎉

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