expose CONNACK properties

Summary: resolves T13155

Reviewers: ivica

Reviewed By: ivica

Subscribers: miljen, iljazovic

Maniphest Tasks: T13155

Differential Revision: https://repo.mireo.local/D26681

include/async_mqtt5/mqtt_client.hpp

@@ -165,6 +165,28 @@ public:
 		});
 	}
 
+	/**
+	 * \brief Retrieves the value of a specific property from the last \__CONNACK\__ packet received.
+	 *
+	 * \details The return type varies according to the property requested.
+	 * For all properties, the return type will be `std::optional` of their respective value type.
+	 * For `async_mqtt5::prop::user_property`, the return type is `std::vector<std::string>`.
+	 *
+	 * \param prop The \__CONNACK\__ property value to retrieve.
+	 *
+	 * \par Example
+	 * \code
+	 *	std::optional<std::string> auth_method = client.connection_property(async_mqtt5::prop::authentication_method); // ok
+	 *	std::optional<std::string> c_type = client.connection_property(async_mqtt5::prop::content_type); // does not compile!
+	 * \endcode
+	 *
+	 * \see See \__CONNACK_PROPS\__ for all eligible properties.
+	 */
+	template <uint8_t p>
+	decltype(auto) connection_property(std::integral_constant<uint8_t, p> prop) {
+		return _svc_ptr->connack_prop(prop);
+	}
+
 	/**
 	 * \brief Assign a \ref will Message.
 	 *