[mqtt-client] add support for enhanced authentication

Summary:
- Relates to T12899
- TODO: support re-authentication

Reviewers: ivica

Reviewed By: ivica

Subscribers: korina

Maniphest Tasks: T12899

Differential Revision: https://repo.mireo.local/D26327

include/async_mqtt5/error.hpp

@@ -486,6 +486,16 @@ requires (cat == connack) {
 	return std::make_pair(valid_codes, len);
 }
 
+template <category cat>
+inline std::pair<reason_code*, size_t> valid_codes()
+requires (cat == auth) {
+	static reason_code valid_codes[] = {
+		success, continue_authentication
+	};
+	static size_t len = sizeof(valid_codes) / sizeof(reason_code);
+	return std::make_pair(valid_codes, len);
+}
+
 template <category cat>
 inline std::pair<reason_code*, size_t> valid_codes()
 requires (cat == puback || cat == pubrec) {