Simplify serializers for custom SSAI descriptions

[MattiasBuelens]

Remove the intermediate SerializedYospaceSsaiDescription data class, and instead assign a custom serializer to the sessionProperties property directly. Also, use a JsonTransformingSerializer to add the extra integration property to the JSON output.

Update the UplynkSsaiDescription serializer to also add the extra integration property. Also, make the serializer internal, and add some unit tests.