Add an oembed example

Basically, we want to return a simple Json response in the example that looks a bit like this:

{
	"version": "1.0",
	"type": "video",
	"provider_name": "YouTube",
	"provider_url": "https://youtube.com/",
	"width": 425,
	"height": 344,
	"title": "Amazing Nintendo Facts",
	"author_name": "ZackScott",
	"author_url": "https://www.youtube.com/user/ZackScott",
	"html":
		"<object width=\"425\" height=\"344\">
			<param name=\"movie\" value=\"https://www.youtube.com/v/M3r2XDceM6A&fs=1\"></param>
			<param name=\"allowFullScreen\" value=\"true\"></param>
			<param name=\"allowscriptaccess\" value=\"always\"></param>
			<embed src=\"https://www.youtube.com/v/M3r2XDceM6A&fs=1\"
				type=\"application/x-shockwave-flash\" width=\"425\" height=\"344\"
				allowscriptaccess=\"always\" allowfullscreen=\"true\"></embed>
		</object>",
}

(https://oembed.com/)

Except, that we won't use the legacy "object" element in the html, but an iframe instead (with the "sandbox" attribute of course). Checkout "WidgetJsonExample" for reference.

@grevil: Let's talk about this first. I don't understand what you mean by example. Shouldn't we instead perhaps add an oembed endpoint submodule, if it helps?

See https://www.hashbangcode.com/article/drupal-9-content-sharing-between-dr...

The oembed json is individual per oembed entity, so an example wouldn't help much, I guess?

But first let's discuss, which advantages this would bring anyway.