Jetpack Boost: Enhance Image and CSS Proxy with CDN and Caching

[pyronaur]

Proposed changes:

• Implement CDN URL handling for image proxy requests.
• Add content type validation and caching for CSS proxy requests.
• Escape error messages for security enhancements.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

N/A

Does this pull request change what data or activity we track or use?

N/A

Testing instructions:

• Use the image guide module to request an image and verify it's served through the Photon CDN.
• Request a CSS file through the CSS proxy and check that it's cached.
• Attempt to proxy a non-CSS file and confirm that they're not loaded

[github-actions]

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Boost plugin:

• Next scheduled release: August 6, 2024.
• Scheduled code freeze: July 29, 2024.

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[dilirity]

Critical css generation works with remote stylesheets.

Just a note below, so we have this mentioned somewhere.

However, Image Guide doesn't (as is to be expected). We're no longer showing correct dimensions of images outside the photon domain.

Now

Before

[donnchawp]

Works as expected. Non image URLs are not proxied by IG Proxy, but images are, including "remote" images off another site.
CCSS files are still generated too.