Add experiment for client-side media processing

[swissspidy]

What?

See #61447 for full context.

Split out from #64278, this adds:

• A new checkbox on the experiments page for client-side media processing
• A first set of REST API changes required for the feature
• Foundation for configuring cross-origin isolation on the editor an adding crossorigin attributes where required

Submitting this PHP-heavy change to make subsequent reviews easier.

Again, this is all behind an experiment. Iterations can be made in subsequent PRs :-)

These changes are all very well tested as part of https://github.com/swissspidy/media-experiments

All these changes are only taking in effect when the experiment is enabled on the settings page. Nothing happens otherwise.

Why?

This is just a small part towards client-side media processing in Gutenberg. See #61447 for full context.

How?

Testing Instructions

There should be a new checkbox on the settings page. No visual changes otherwise.

Testing Instructions for Keyboard

N/A

Screenshots or screencast

N/A

[github-actions]

Flaky tests detected in 7278d79.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/10548153719
📝 Reported issues:

• [Flaky Test] Should insert content using the global inserter #57525 in /test/e2e/specs/widgets/editing-widgets.spec.js
• [Flaky Test] Sorting #64430 in /test/e2e/specs/site-editor/new-templates-list.spec.js

[swissspidy]

Note: the only PHP unit test that's failing is because of #64743. Once that PR is merged, it should be fine.

[github-actions]

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Co-authored-by: swissspidy <swissspidy@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: spacedmonkey <spacedmonkey@git.wordpress.org>
Co-authored-by: TimothyBJacobs <timothyblynjacobs@git.wordpress.org>
Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>

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[felixarntz]

@swissspidy This looks great so far. Left a number of comments with little things to improve, but nothing major.

[TimothyBJacobs]

This looks good to me from a REST API perspective.

[swissspidy]

@spacedmonkey Could you please re-review after the latest changes? Your stale review is blocking the merge.

[adamsilverstein]

Fantastic! Left a few questions.

[felixarntz]

Looks great to me!

[spacedmonkey]

@swissspidy Provided a review already, but I comments seem to losted in the shuffle. Can you take a look at my last comments.

[swissspidy]

@spacedmonkey All comments were addressed and/or resolved, that's why I am asking for your re-review to unblock this merge. Further changes can be easily done in follow-up PRs, as everything here is behind an experimental flag anyway.

[spacedmonkey]

LGTM. Good work @swissspidy