Redundant data type descriptors on data stream data points

[Whathecode]

The polymorphic type descriptor added to data when it is used as part of data streams is redundant.

It duplicates the data stream data type](https://github.com/cph-cachet/carp.core-kotlin/blob/develop/carp.data.core/src/commonTest/resources/test-requests/DataStreamService/1.1/DataStreamServiceTest/appendToDataStreams_succeeds.json#L42):

            "batch": [
                {
                    "dataStream": {
                        "studyDeploymentId": "3de35fb5-c39e-4fea-8ccc-3a70ca618ce7",
                        "deviceRoleName": "Device",
                        "dataType": "dk.cachet.carp.stubpoint"         // DEFINES TYPE
                    },
                    "firstSequenceId": 0,
                    "measurements": [
                        {
                            "sensorStartTime": 0,
                            "data": {
                                "__type": "dk.cachet.carp.stubpoint",       // REDUNDANT; ALWAYS THE SAME
                                "data": "Stub"
                            }
                        }
                     ]
                  }
               ]

A custom serializer could potentially omit all the __type information.