feat: add cookie-related API to Message (#491)

Add `Message::getCookie()` and `MethodReply::getReplyCookie()` functions, based on underlying sd-bus cookie functions. They can be useful when pairing method call messages and method call reply messages.

---------

Co-authored-by: Dylan Howey <dylan.howey@evgo.com>
Co-authored-by: Stanislav Angelovič <stanislav.angelovic@protonmail.com>

include/sdbus-c++/Message.h

@@ -215,6 +215,7 @@ namespace sdbus {
         const char* getSender() const;
         const char* getPath() const;
         const char* getDestination() const;
+        uint64_t getCookie() const;
         // TODO: short docs in whole Message API
         std::pair<char, const char*> peekType() const;
         bool isValid() const;
@@ -296,6 +297,7 @@ namespace sdbus {
     public:
         MethodReply() = default;
         void send() const;
+        uint64_t getReplyCookie() const;
     };
 
     class Signal : public Message