uart-service: Implement notification support

[kurtjd]

The UART service needs a way to handle asynchronous notifications to the host (as in, support a service getting the host's attention outside of the typical request -> response flow).

This will likely be handled in-band (through some sentinel value) so as to not need additional wires, but could be challenging since binary data (not just ASCII) is transferred between host and EC.

[kurtjd]

Depends on #733 first. As in, first we need to come up with a way to represent notifications generically then we can figure out how to represent that on the wire for uart.