Support SHORT_STRING data type for symbolic tag messaging

[greg9504]

The SHORT_STRING data type is a single byte character string. It is sent with the first byte being the length, followed by the string data. Supporting symbolic (Rockwell) tags with this data type is relatively straightforward. Also add support for USINT (unsigned 8bit data) Code changes here:

https://github.com/greg9504/ST-node-ethernet-ip/tree/Add_Additional_DataTypes_Support

There was also a change for writing SINT data, the code used Buffer.writeUInt8 when I think it should have used Buffer.writeInt8.

Further testing should probably be done, as I've only be able to test using the Micro850 simulator in Connected Components Workbench.

Tested with CCW version 22 PLC - 2080-LC50-48QWB-SIM (Micro850 Simulator, not actual hardware)