Proxy content via public-api, even when we're on a simple site.

[devicenull]

Related to #87642

Proposed Changes

In order to solve #87642, we need to proxy any images in the media area via the public API.

This solves two main cases:

1. Where the primary redirect is not yet pointed to us via DNS
2. Where a service like Cloudflare is in front of the primary redirect, and blocks "hotlinking" images

These cases both used to work fine, as media was served from *.files.wordpress.com regardless of the actual domain being used. Once this was been moved to use wp-content/uploads (in order to fix issues when third party cookies are blocked), the two cases above (and probably some others we don't know about) stopped working.

This will not work until D149138-code is deployed

Testing Instructions

• Requires a public-api sandbox with the changes from D149138-code applied
• Open a media library like http://calypso.localhost:3000/media/meltbeforefailure.net
• Verify images appear
• Inspect an image, verify the src is "blob:http://calypso.localhost" instead of a wp-content/uploads URL

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• [n/a] Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• [n/a] Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• [n/a] For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[devicenull]

D149138-code has been deployed

[devicenull]

duplicate of #90895