Use CDN for block directory assets

[tellyworth]

This changes the block directory REST API to use CDN urls for script and CSS assets. It also allows for absolute URLs when returned by the remote API.

How has this been tested?

Unit tests, including new additions to WP_REST_Block_Directory_Controller_Test::test_prepare_item().

Types of changes

This also supports fully qualified URLs in future, and incorporates some raw escaping.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[dd32]

Based purely on a visual review, this is what I had in mind myself.

Note: There's two spaces before each PHP_URL_SCHEME which should be compressed.