Add credentialless attribute to oEmbed iframes

Problem/Motivation

This could be an alternative solution to #3489437: Evaluate adding the sandbox attribute to the oEmbed iframe.

This is what the attribute does according to MDN:

This results in the documents inside the credentialless <iframe> being loaded using new, ephemeral contexts — those contexts don't have access to the data associated with their origins; for example cookies and localStorage.

So this could prevent the media in the iframe from accessing anything sensitive.
I'm not sure most oEmbed content benefits from having access to the user session (except for ad/tracking purposes I guess).
But I could see use cases for it so this should probably be optional.

The main problem is that it is currently not supported by Safari and Firefox: https://caniuse.com/wf-iframe-credentialless

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet