[scala] Change ScalaAggregateOperator to use TypeSerializer

Before it was using TypeSerializerFactory. This led to problems with the
user code class loader and is not necessary.
We could maybe. If we enhanced TypeSerializer to have a method createInstance(...) that can create an instance with the given field values. This is only possible for a small subset of the types that we support: The Java Tuple Types, since we control their constructors. And Scala Case Classes, since we assume that the fields are exactly the constructor arguments, as is customary for Case Classes.

We would then need to add createInstance(...) methods to all other TypeSerializers that throw an Exception telling the user that instance creation with field values is not supported.