[mqtt-client] coroutine & callback examples

Summary:
related to T12804
	- added asio::no_recovery to list of possible error codes
	- added examples with asio::use_awaitable and callbacks as completion tokens

Reviewers: ivica

Reviewed By: ivica

Subscribers: miljen, iljazovic

Differential Revision: https://repo.mireo.local/D26355

include/async_mqtt5/error.hpp

@@ -161,6 +161,12 @@ public:
 	/// Move constructor.
 	reason_code(reason_code&&) = default;
 
+	/// Copy assignment operator.
+	reason_code& operator=(const reason_code&) = default;
+
+	/// Move assignment operator.
+	reason_code& operator=(reason_code&&) = default;
+
 	/**
 	 * \brief Indication if the object holds a Reason Code indicating an error.
 	 *