[C++] Add compute kernel to get MapArray value for given key

Given a "map", an obvious operation is to get an item corresponding to a key. The idea here is to create a kernel that does this for each map in the array.

IIRC MapArray isn't guaranteed to have unique keys. So one version would return an array of ItemType by returning the first of last item for a given key. Yet another version could return a ListType containing all matching items.