feat: introduce support for struct-as-dict serialization (#459)

This extends the functionality of SDBUSCPP_REGISTER_STRUCT macro.

It now provides functionality for serializing a user-defined struct as an a{sv} dictionary, and for deserializing an a{sv} dictionary into a user-defined struct. The former one is achieved by decorating the struct with sdbus::as_dictionary(struct), the latter one is an automatic behavior -- when sdbus-c++ is asked to deserialize into a struct but the data in the message is of type a{sv}, then the dict-to-struct deserialization is performed automatically.

There are some aspects of behavior in the serialization/deserialization functionality that can be customized by the client. Newly introduced SDBUSCPP_ENABLE_NESTED_STRUCT2DICT_SERIALIZATION and SDBUSCPP_ENABLE_RELAXED_DICT2STRUCT_DESERIALIZATION macros serve the purpose.

tests/integrationtests/TestAdaptor.cpp

@@ -246,6 +246,11 @@ void TestAdaptor::sendLargeMessage(const std::map<int, std::string>& /*collectio
     //printf("Adaptor: got collection with %zu items", collection.size());
 }
 
+std::map<std::string, sdbus::Variant> TestAdaptor::returnDictionary(const std::map<std::string, sdbus::Variant>& dict)
+{
+    return dict;
+}
+
 std::string TestAdaptor::state()
 {
     return m_state;