REST API: Use `wp.apiRequest` helper to build URLs for media embed requests

[jnylen0]

Follow-up to #40919.

Let's explore getting rid of the oEmbedProxyUrl ​property used to pass a REST API URL down to wp-admin code that calls out to this endpoint (​example). We can use the helper library introduced in #40919 instead:

wp.apiRequest( {
    namespace: 'oembed/1.0',
    path: '/proxy',
    // ...
} );

This helper will allow us to eliminate all rest_url calls passed to client code through script localization. This will eventually lead to a lot of duplication, but it seems like it should be very clear from the requesting code which API endpoint is being called.

[nezhar]

I have done the path a described above. How can I test this actually? I tried by embedding a Youtube video, but I don't think it uses the action that got modified.