proxy: add IProxy::getConnection() (#179)

This provides access to the proxy's bus connection so code using
the proxy does not need to store an external reference to it.

A matching function is already available in IObject.

src/Proxy.h

@@ -60,6 +60,7 @@ namespace sdbus::internal {
         void finishRegistration() override;
         void unregister() override;
 
+        sdbus::IConnection& getConnection() const override;
         const std::string& getObjectPath() const override;
 
     private: