e2e-demo: Demonstrate comms from host via USB-to-UART to EC

[kurtjd]

After the uart-service is implemented (see OpenDevicePartnership/embedded-services#605), demonstrate its use by sending info to and from the test app to the EC over UART.

As an initial proof-of-concept, a USB-to-UART device will be used, with the USB end connected to host and the UART end connected to EC directly. A small crate will be written which uses the existing MCTP crate to wrap data in a MCTP packet and send it over a given serial/COM port. This crate will then be integrated into ec-test-app as a new Source which can be activated via feature flag so the app sends its data through this instead of the ACPI driver.