Provide provider ID as variable for Twig template

Problem/Motivation

When trying to override the video-embed-iframe.html.twig template to wrap a contextual consent around the iframe it is a bit tricky to extract the provider ID. There is only the {{ theme_hook_original }} which is e.g. video_embed_iframe__youtube, so in order to extract youtube you would need to do ugly replacing like:

{% set provider = theme_hook_original|replace({'video_embed_iframe__': ''}) %}

Proposed resolution

Add a provider variable via preprocess so no ugly replacing is needed.