Implement Becker Port for FUJINET-PC

[tschak909]

There are emulators that support connection to external resources over TCP sockets. An example of this is the "Becker Port" for TRS-80 Color Computer emulators. This is supported in MAME, VCC, and XRoar.

To support this, the UARTManager class needs to be generalized slightly, to handle both serial port devices, and TCP sockets.

The current UARTManager class can be seen here:
https://github.com/FujiNetWIFI/fujinet-firmware/blob/master/lib/hardware/fnUART.cpp
https://github.com/FujiNetWIFI/fujinet-firmware/blob/master/lib/hardware/fnUARTUnix.cpp
https://github.com/FujiNetWIFI/fujinet-firmware/blob/master/lib/hardware/fnUARTWindows.cpp

Note: The ESP_PLATFORM define is used to differentiate between the ESP32 and Fujinet-PC build.

This will be considered working, when one of the above emulators can connect to a running instance of FujiNet-PC, and be able to use FujiNet functions.