Feature Request: RTU Over TCP Support for QModbusClient

Hi,

The current QModbusClient implementation supports two modes of operation: RTU and TCP, which cover the most common use cases. However, there are scenarios where sending raw RTU frames over Ethernet (via a socket) is required. This is common in applications where devices such as Moxa NPort Servers are used, which forward raw socket data directly to their serial ports. In such cases, there is no need to encapsulate the RTU frame according to the ModbusTCP specification.

I recommend adding a third mode to QModbusClient, RTUOverTCP. This mode would allow the client to behave like QModbusRtuSerialClient, but instead of using serial ports, it would send raw Modbus RTU frames over a defined socket connection.

As an automation engineer with years of experience integrating various Modbus devices, I've frequently encountered the need to bridge RTU frames over Ethernet. Unfortunately, the current Qt Modbus implementation lacks this functionality, even though it's a common requirement in industrial automation environments.

Thank you for considering this feature enhancement.