Recommendations: allow multi value profile param as recommendation key

[#1]

Today you can set a custom recommendation key (profile param) for existing algoritms like "people who bought this, also bought that". The recommendation key can only hold 1 value (1 entity id = 1 product id). For use cases where more than 1 entity id should serve as recommendation key this is a short coming. For example: wish lists. If a user has created a wish list on an e-commerce website, it would be great to create recommendations based on the content of the entire list (just like with the content of the entire cart).