GET    /V1/products/:sku/downloadable-links
GET    /V1/products/:sku/downloadable-links/samples
POST   /V1/products/:sku/downloadable-links
PUT    /V1/products/:sku/downloadable-links/:id
DELETE /V1/products/downloadable-links/:id
POST   /V1/products/:sku/downloadable-links/samples
PUT    /V1/products/:sku/downloadable-links/samples/:id
DELETE /V1/products/downloadable-links/samples/:id
