PortNamespace supports `serializer`

[superstar54]

One can pass a serializer parameter to spec.input to automatically serialize the input into AiiDA data, such as to_aiida_type. For the moment, if I pass it into spec.input_namespace, it will raise an error,

spec.input_namespace("kwargs", valid_type=Data, required=False, serializer=to_aiida_type)

TypeError: PortNamespace.__init__() got an unexpected keyword argument 'serializer

It would be great to have this feature for spec.input_namespace too.