Vimeo embedded videos and ExternalVideo type changes

Shopify now supports adding embedded Vimeo videos to products, leveraging the ExternalVideo object.

Many ExternalVideo API implementations assume that all external video objects represent YouTube videos. To help determine where the video is hosted, we're introducing the new field host to ExternalVideo that will return a MediaHost enum value (currently VIMEO or YOUTUBE).

Additionally, we're introducing two new MediaErrorvalues EXTERNAL_VIDEO_EMBED_NOT_FOUND_OR_TRANSCODING and EXTERNAL_VIDEO_EMBED_DISABLED for Vimeo error handling.