Implementing and testing for non-standard transformers

[kmyram]

Feature description

Implementing non-standard/optional/experimental transformers as based in the discovery of #4213. This provides the actual transformers based on the findings in the discovery. For now, the scope is not determined and subsequently neither is AC.

Relates to #4213, #4215, #4216
Related to #958

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[schlessera]

This is superseded by these issues:

1. ServerSideRendering transformer with the following functionality:
   • Rewrite media, sizes, and heights attributes for SSR
2. Minify transformer with the following functionality:
   • Minify HTML
   • Strip comments
   • Trim whitespace in body text nodes
3. BrowserHints transformer with the following functionality:
   • Preload images
4. BlurredPlaceholders transformer
   • Add blurred placeholders for images and video posters