Jetpack Boost: Enhance Image and CSS Proxy with CDN and Caching (#38252)

* Make sure CSS proxy only attempts to fetch CSS

* Pass image guide requests through photon

* esc_html the error message

* Add plugins/boost changelog entry.

* excess space

* wp_parse_url

* remove undefined variable

* Make sure that $css is set

* return void

* Remove escess return

* Use `wp_safe_remote_get`

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/9869880298

Upstream-Ref: Automattic/jetpack@3ce744a