SSDM-6993 : add support for downloading folders

source/java/ch/ethz/sis/filetransfer/Chunk.java

...va/ch/ethz/sis/filetransfer/DefaultChunkDeserializer.java

...java/ch/ethz/sis/filetransfer/DefaultChunkSerializer.java

...java/ch/ethz/sis/filetransfer/DownloadClientDownload.java

source/java/ch/ethz/sis/filetransfer/DownloadSessionId.java

...ava/ch/ethz/sis/filetransfer/FileSystemChunkProvider.java

...ava/ch/ethz/sis/filetransfer/FileSystemDownloadStore.java

source/java/ch/ethz/sis/filetransfer/IDownloadItemId.java

source/java/ch/ethz/sis/filetransfer/IUserSessionId.java

...est/java/ch/ethz/sis/filetransfer/DownloadClientTest.java

...Test/java/ch/ethz/sis/filetransfer/TestChunkProvider.java

...est/java/ch/ethz/sis/filetransfer/TestDownloadItemId.java

...ethz/sis/filetransfer/TestDownloadItemIdDeserializer.java

...h/ethz/sis/filetransfer/TestDownloadItemIdSerializer.java

...Test/java/ch/ethz/sis/filetransfer/TestDownloadStore.java

sourceTest/test-files/testFolder/testFile3.txt

sourceTest/test-files/testFolder/testFile4.txt

sourceTest/test-files/testFolder/testFolder2/testFile5.txt