Preload the image-set background images

[piotrbak]

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Describe the bug
As a part of 3.16 scope we need to preload the image-set background images when they're detected as LCP.

Expected behavior
Input:

background-image: image-set( "cat.png" 1x, "cat-2x.png" 2x);
background-image: image-set( url("cat.png") 1x, url("cat-2x.png") 2x);

or

background-image: -webkit-image-set( "cat.png" 1x, "cat-2x.png" 2x);
background-image: -webkit-image-set( url("cat.png") 1x, url("cat-2x.png") 2x);

Output:

<link rel="preload" as="image" imagesrcset="cat.png 1x, cat-2x.png 2x" fetchpriority="high">

Acceptance Criteria (for WP Media team use only)
LCP

1. Images are preloaded using the image URL from the image-set declaration, they'll be added to the imagesrcset attribute
2. fetchpriority="high" is added to the preload markup
3. Images are excluded from LL Background Images feature

Above the fold

1. Images are excluded from LL Background Images feature

[piotrbak]

@DahmaniAdame @benorfaz There's also another background image-set to be handled:
-webkit-image-set

There's a possible edge case when image-set and -webkit-image-set are different images, then we'll preload both of them while without WP Rocket it'll load only one. I guess this is acceptable one.

CC @Mai-Saad

[Mai-Saad]

Note: related test page on e2e
https://e2e.rocketlabsqa.ovh/lcp_bg_reponsive_imgset_template
https://e2e.rocketlabsqa.ovh/lcp_bg_responsive_webkit_template

[remyperona]

PHP part is handled for this case.

[jeawhanlee]

Expected data structure in DB for LCP:

   {
     "type": "bg-img-set",
     "bg_set": [
       {"src": "cat.png 1x"},
       {"src": "cat-2x.png 2x"}
     ]
   }

Same will apply for ATF.