SSDM-774 Allow to assign a representative thumbnail for a dataset

...penbis/generic/shared/basic/dto/ServiceVersionHolder.java

...sd/openbis/dss/etl/dto/api/IImageGenerationAlgorithm.java

...x/cisd/openbis/dss/etl/dto/api/SimpleImageDataConfig.java

...bis/dss/etl/dto/api/impl/EmptyImageCreationAlgorithm.java

...openbis/dss/etl/dto/api/impl/ImageDataSetInformation.java

...i/impl/MaximumIntensityProjectionGenerationAlgorithm.java

.../etl/jython/v2/ImagingDataSetRegistrationTransaction.java

...nbis/dss/etl/jython/v2/SimpleImageDataSetRegistrator.java