Adds std::initializer_list overloads to Promise::any() & Promise::all()

Using initializer lists (i.e. {myPromise, anotherPromise}) does not work with template type deduction. Hence, one has to create explicit overloads for std::initializer_list<T> to avoid the need to explicitly state the template type.

This MR adds those overloads for Promise::all() and Promise::any().