Create a product variant

Creates a new product variant for the given merchant. If a product-variant already exists for the provided variant_id, the existing product-variant is returned without modification. If a variant_id is not provided and a product-variant already exists for the same SKU & barcode combination provided, the existing product-variant is returned without modification. Otherwise, a new product-variant will be created. Metadata is persisted in the same database transaction as the variant. If a source image URL is provided, CDN image processing is scheduled asynchronously after creation.