Changes to Create an in stock subscription

added the new required request property data/relationships/customer/data

added the new required request property data/relationships/market/data

added the new required request property data/relationships/sku/data

request property data/type was restricted to a list of enum values

removed the request property data/relationships/customer/id

removed the request property data/relationships/customer/type

removed the request property data/relationships/market/id

removed the request property data/relationships/market/type

removed the request property data/relationships/sku/id

removed the request property data/relationships/sku/type

the type request property default value in_stock_subscriptions was removed

added the new in_stock_subscriptions enum value to the request property data/type

added the media type application/vnd.api+json for the response with the status 201