Lantronix has few products which are in size of Ethernet jack. Expensive ~70USD, but nice black box solution. Lantronix also has driver for windows to make it look like a serial port.
There will be no off the self single chip solution. you can make one in few hours. Take any Ethernet dev board may be microchip or even a Raspberry, have socket open on Ethernet and use a serial port to push data in and out of socket. Make it appear like a serial port in windows would need a driver. Doing custom driver in Windows is hard.