Add custom provider to existing remote video media type

@zebda You'd need to change the media source of the media type, which core doesn't allow: #2928256: Users shouldn't be able to change the media source plugin after the media type is created → .

That said, when I look at the commit, I'm seeing the only change was in the MediaTypeForm class. I don't see anything from stopping you from changing the media source programmatically. You might even be able to make the change by updating the media type's config entity in core's configuration management UI (e,g. /admin/config/development/configuration/single/export and /admin/config/development/configuration/single/import). Definitely test in non-prod first.