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>

tests/integrationtests/TestFixture.h

@@ -50,6 +50,8 @@
 
 namespace sdbus { namespace test {
 
+inline const uint32_t ANY_UNSIGNED_NUMBER{123};
+
 class BaseTestFixture : public ::testing::Test
 {
 public: