fix: add missing c-tor for nesting Variants (#467)

This allows nesting variants, i.e. allows a variant to contain another variant as its value. Until now, there was no such possibility, since the default generated copy constructor would be invoked which would create a copy of source variant instead of embed the source variant as a value in the destination variant. The default generated copy constructor is kept, for it makes sense too, but a new tag-based overload is added for embedding the source variant into the destination variant.

tests/integrationtests/TestAdaptor.cpp

@@ -266,6 +266,16 @@ void TestAdaptor::action(const uint32_t& value)
     m_action = value;
 }
 
+sdbus::Variant TestAdaptor::actionVariant()
+{
+    return m_actionVariant;
+}
+
+void TestAdaptor::actionVariant(const sdbus::Variant& value)
+{
+    m_actionVariant = value;
+}
+
 bool TestAdaptor::blocking()
 {
     return m_blocking;