[Feature] Support Keyboard Input Mapping to 8 In-Game Buttons

### Is your feature request related to a problem?

I'm building a physical 8-key attachment for tablets that works by simulating keyboard presses (via USB). To make it usable with AstroDX, please add support for mapping arbitrary keyboard keys to the 8 in-game touch buttons.

### Describe the feature

A simple config file or in-app settings would suffice. Suggested default layout: A S D F for left 4, J K L ; for right 4, but user-customizable is ideal.

### Reason for adding

I'm building a physical 8-key attachment for tablets that works by simulating keyboard presses (via USB). To make it usable with AstroDX, please add support for mapping arbitrary keyboard keys to the 8 in-game touch buttons.

### Example(s)

_No response_

### Additional context

_No response_