AUX GPIO Event Trigger ("Interrupts")

The values configured as digital *inputs* need to be able to send Harp `EVENT` reply messages if their outputs rise, fall, or change.

We need to be able to configure which of the digital inputs will emit these messages, and on what condition they will emit. We can do this with 3 registers:
```cpp
{
uint8_t AuxGPIOInputEvent;
uint8_t AuxGPIORise;
uint8_t AuxGPIOFall;
}
```

The `AuxGPIOInputEvent` will configure which GPIOs will emit a message, and the following two registers will configure what pin change (or both) will emit the EVENT.

These events should emit from the `AuxGPIOInputEvent` or `AuxGPIOState` register.