Minimal c interface

Summary

See https://gitlab.indiscale.com/caosdb/customers/lfpb/management/-/issues/404

• Minimal C Interface
• Configuration via json files
• Renaming of many classes and functions (In order to comply with best-practices as formulated by https://google.github.io/styleguide/cppguide.html)

Integration tests are in caosdb-cppinttest!1 (merged)

Focus

• Please take a look at the c-interface functions:
  • I the naming consistent?
  • I see one problem regarding the destructors: When a client uses caosdb_connection_connection_manager_get_default_connection to get the default connection, the destructor caosdb_connection_delete_connection should not be called, because the connection object is actually owned by the ConnectionManager. Maybe, we should prevent the deletion of the wrapped connection somehow, when the connection is owned by the ConnectionManager?
• ConfigurationManager and ConnectionManager:
  • These two singletons are tightly coupled. When the ConfigurationManager is being reset, also the ConnectionManager resets. Is this a good behavior?
• There is currently no error/exception handling in the c-interface. I will add this in https://gitlab.indiscale.com/caosdb/customers/lfpb/management/-/issues/390

Test Environment

Please try and setup a client (e.g. the ccaosdbcli) with a json configuration file.

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• All automated tests pass
• Reference related Issues
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  • Intent of new code
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Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md
• The test environment setup works and the intended behavior is reproducible in the test environment
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include/caosdb/constants.h.in

@@ -22,11 +22,15 @@
 
 #ifndef CAOSDB_CONSTANTS_H
 #define CAOSDB_CONSTANTS_H
+#ifdef __cplusplus
 namespace caosdb {
+#endif
 // clang-format off
-constexpr int LIBCAOSDB_VERSION_MAJOR = @libcaosdb_VERSION_MAJOR@;
-constexpr int LIBCAOSDB_VERSION_MINOR = @libcaosdb_VERSION_MINOR@;
-constexpr int LIBCAOSDB_VERSION_PATCH = @libcaosdb_VERSION_PATCH@;
+const int LIBCAOSDB_VERSION_MAJOR = @libcaosdb_VERSION_MAJOR@;
+const int LIBCAOSDB_VERSION_MINOR = @libcaosdb_VERSION_MINOR@;
+const int LIBCAOSDB_VERSION_PATCH = @libcaosdb_VERSION_PATCH@;
 // clang-format on
+#ifdef __cplusplus
 } // namespace caosdb
 #endif
+#endif