GaudiConfig2: add clone for Configurable

Add a clone method to the Configurable class and tests. The implementation is trivial but without it the user would have to access the "private" _properties.

GaudiConfiguration/python/GaudiConfig2/_configurables.py

 #####################################################################################
-# (c) Copyright 1998-2022 CERN for the benefit of the LHCb and ATLAS collaborations #
+# (c) Copyright 1998-2023 CERN for the benefit of the LHCb and ATLAS collaborations #
 #                                                                                   #
 # This software is distributed under the terms of the Apache version 2 licence,     #
 # copied verbatim in the file "LICENSE".                                            #
@@ -273,6 +273,10 @@ class Configurable(ConfigMetaHelper):
     def getDefaultProperty(cls, name):
         return cls._descriptors[name].default
 
+    def clone(self, newname=None):
+        """Clone instance with all its properties."""
+        return self.__class__(newname, **self._properties)
+
     def merge(self, other):
         """
         Merge the properties of the other instance into the current one.