Add support for Modbus RTU over UDP

[zsoltyfm]

I would recommend for consideration that it would be worthwhile to add the possibility to transmit modbus RTU frames via TCP and UDP ethernet network.

I often come across that the device used is a standard ethernet-rs232-rs485 gateway and therefore does not handle the modbus TCP protocol by default, but if normal RTU frames are transmitted and received on an Ethernet network, then communication can also be implemented in this way.

This would be an often useful feature.

https://www.modbustools.com/modbus_poll.html

[per1234]

Thanks for your suggestion @zsoltyfm. As you are aware, there is already a request for TCP support: #123 so this request is redundant in that respect. I am not knowledgeable on this subject, but I suppose we can treat the request for UDP support as unique and use this issue to track that aspect exclusively.