ENH: Test datatypes and values in Extern C interface

Integration tests for https://gitlab.indiscale.com/caosdb/customers/lfpb/management/-/issues/420 and caosdb-cpplib!13 (merged)

Almost solely whitespace changes that my clang-tidy performed, the only real changes concern the way strings are "returned" by the Extern C interface.

test/test_transaction.cpp

@@ -781,7 +781,7 @@ TEST_F(test_transaction, test_file_up_n_download) {
  */
 TEST_F(test_transaction, test_full_workflow) {
   const auto &connection =
-      caosdb::connection::ConnectionManager::GetDefaultConnection();
+    caosdb::connection::ConnectionManager::GetDefaultConnection();
 
   // ######  Create Data Model  ######
   Entity dataRT;