Improve TaskResult Rest handling

- fix issues related to raw results (byte array), add a isRaw information to know that the result is in byte array format and should not be deserialized using an ObjectInputStream
- use Base64 conversion for TaskResult's serialized objects in dozer mappings
- Enable variable retrieval from the rest TaskResult (TaskResultData). Variables are accessible either in serialized/base64 format or in text format (toString)