Provide a mechanism for arbitrary input

While there are existing plans for arbitrary joystick and keyboard support, we should also consider some simple way of getting custom inputs (from software programs (e.g. a user's script on their control station computer) and hardware (e.g. anything that has some concept of buttons)) to work as triggers in Cockpit.

[This project](https://discuss.bluerobotics.com/t/the-lights-controller-that-no-one-needs-but-might-help-with-custom-controller-projects/13825) provides relevant context - the user wanted to make a custom controller with just two buttons for controlling some lights through QGC, and they needed to use a Joystick library to do so, and set up a fake calibration process just so QGC would accept inputs from it.

Start by reviewing the existing plans for arbitrary joystick and keyboard support, then read the linked Blue Robotics discussion for the custom-controller use case. Define what interface would let software scripts and simple button hardware trigger Cockpit actions without pretending to be a calibrated joystick; the issue is done when that mechanism is specified and supported.

Written by the indexing model from the issue text.