feat: Allow specyfing string subtype for product ids

[AmauryLiet]

Issue

I want to protect myself by restraining the acceptable skus in my application (either with type ItemSku = 'sku1' | 'sku2' | ... or enum ItemSku { sku1 = 'sku1', sku2 = 'sku2', ... }

However, getProducts returns products with productId typed as string

Fix

getProducts now infers the type of productId from its input

This is backward compatible as the typing is optionnal and defaults to the current string

[hyochan]

LGTM!