Add implementation for user creation/deletion/retrieval

include/CMakeLists.txt

include/caosdb/acm/role.h

include/caosdb/acm/user.h

include/caosdb/connection.h

include/caosdb/file_transmission/download_request_handler.h

include/caosdb/file_transmission/upload_request_handler.h

src/CMakeLists.txt

src/caosdb/acm/permission_rule.cpp

src/caosdb/acm/role.cpp

src/caosdb/acm/role_impl.h

src/caosdb/acm/user.cpp

src/caosdb/acm/user_impl.h

src/caosdb/connection.cpp

src/caosdb/file_transmission/download_request_handler.cpp

src/caosdb/file_transmission/upload_request_handler.cpp

src/caosdb/transaction.cpp

src/caosdb/unary_rpc_handler.cpp

test/CMakeLists.txt

test/test_connection.cpp

test/test_role.cpp