Allow embedding of galleries using core media

Problem/Motivation

I wont to embed my media galleries in content using media embed.

Problem

Media gallery does not provide a media_gallery media entity.

Proposed resolution

• Provide @MediaSource plugin for Gallery
• Provide (unidirectional) syncing of media galleries with the media `media_field_data` on hook_media_gallery_update

At least that's my approach so far. Feedback is welcome.