Add license headers to new files

include/caosdb/file_transmission/download_request_handler.h

include/caosdb/file_transmission/file_error.h

include/caosdb/file_transmission/file_reader.h

include/caosdb/file_transmission/file_writer.h

...e/caosdb/file_transmission/register_file_upload_handler.h

include/caosdb/file_transmission/upload_request_handler.h

include/caosdb/transaction_handler.h

include/caosdb/unary_rpc_handler.h

src/caosdb/file_transmission/download_request_handler.cpp

src/caosdb/file_transmission/file_reader.cpp

src/caosdb/file_transmission/file_writer.cpp

...caosdb/file_transmission/register_file_upload_handler.cpp

src/caosdb/file_transmission/upload_request_handler.cpp

src/caosdb/transaction_handler.cpp

src/caosdb/unary_rpc_handler.cpp