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.

include/sdbus-c++/IProxy.h

@@ -267,6 +267,13 @@ namespace sdbus {
          */
         [[nodiscard]] PropertySetter setProperty(const std::string& propertyName);
 
+        /*!
+         * @brief Provides D-Bus connection used by the proxy
+         *
+         * @return Reference to the D-Bus connection
+         */
+        virtual sdbus::IConnection& getConnection() const = 0;
+
         /*!
          * @brief Returns object path of the underlying DBus object
          */