[Core] Add methods to retrieve available sub-items and handle not found errors in Registry and RegistryItem

[loumalouomega]

📝 Description

Add methods to retrieve available sub-items and handle not found errors in RegistryItem. Until now the error message when trying to retrieve an item were not meaningful, were just displating that the given name was not available, not displaying the list of potential candidates (as done in "legacy" KratosComponents). This message would be very helpful, because sometimes the problem may come from some typo of case sensitivity.

For example:

Registry::GetValue<Process>("Processes.KratosMultiphysics.output_process.Prototype"); // Instead of OutputProcess

Then:

C++ exception with description "Error: The item "Processes.KratosMultiphysics.output_process.Prototype" is not found in the registry. The item "KratosMultiphysics" does not have "output_process". The available objects are: 
                OutputProcess
                ApplyRayCastingProcess<3>
                ApplyRayCastingInterfaceRecognitionProcess<3>
                ApplyRayCastingInterfaceRecognitionProcess<2>
                ApplyRayCastingProcess<2>
                Process

Adding new tests.

🆕 Changelog

• Add methods to retrieve available sub-items and handle not found errors in RegistryItem
• Refactor test_registry.cpp to use nested namespace Kratos::Testing and add exception expectation for missing sub-item retrieval
• Add tests for registry item retrieval and expected failures in Python registry
• Implement NotFoundError method for improved error handling in Registry and RegistryItem
  44df6de
  Add exception expectation for invalid registry item retrieval in tests

[roigcarlo]

Foine