Support ASIO_STANDALONE in mqtt_cpp

[brinkqiang]

The mqtt_cpp library is a great tool for working with MQTT. However, it currently does not support ASIO_STANDALONE. This would be a valuable addition to the library, as it would allow users to build MQTT applications without having to depend on Boost.

ASIO_STANDALONE is a standalone version of ASIO that does not require Boost. This makes it a more lightweight and portable option for developers.

I would like to request that the mqtt_cpp developers consider adding support for ASIO_STANDALONE. This would make the library more versatile and easier to use for a wider range of developers.