Passing currency/language headers to addProductToCart function.

[rishi-ayata]

As of now, currency and language are set at gateway level during Motlin gateway initialization. If the gateway is initialized based on a singleton pattern, it's difficult to reuse the same gateway across multiple apis from a storefront that handles multiple currencies and languages. One solution would be to use a request scoped gateway initialization which is an expensive approach as the number of requests gets higher. An alternative way is to pass language as well as currencies as optional parameters with addProduct or similar APIS which makes it more flexible to use.