Add expansion parameter to the customer profile endpoint

[nikola-mladenovic]

This can be handy, in cases where customer custom type contains references to other objects, like in Sunrise example, we keep myStore channel ID. It'd be better to retrieve complete store details (channel object), along with user profile on my accounts page (expansion = ["custom.fields.myStore"]).

In order to achieve that, we should first add expansion to the customer profile endpoint in the SDK.