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.h

@@ -85,6 +85,7 @@ protected:
     void doPrivilegedStuff() override;
     void emitTwoSimpleSignals() override;
     void sendLargeMessage(const std::map<int, std::string>& collection) override;
+    std::map<std::string, sdbus::Variant> returnDictionary(const std::map<std::string, sdbus::Variant>& dict) override;
 
     uint32_t action() override;
     void action(const uint32_t& value) override;
@@ -147,6 +148,7 @@ protected:
     void doPrivilegedStuff() override {}
     void emitTwoSimpleSignals() override {}
     void sendLargeMessage(const std::map<int, std::string>&) override {}
+    std::map<std::string, sdbus::Variant> returnDictionary(const std::map<std::string, sdbus::Variant>&) override { return {}; }
 
     uint32_t action() override { return {}; }
     void action(const uint32_t&) override {}