Async matrix scanning, reduce power consumption

[HaoboGu]

Closes #24

Background

In current implementation, the keyboard matrix is continuously scanned, which leads to high power consumption, and low battery life in BLE situation.

Implementation

This PR adds a RMK feature, async_matrix, which is an async way for matrix scanning. When the the keyboard is idle, the matrix will not be scanned. Instead, all the output pins are set to high level, and the keyboard will monitor the input pins using wait_for_low(), which is actually the Wait trait in embedded-hal-async. In embassy's implementation, wait_for_low() is low-power by default. So this would reduce the idle power consumption.

Notes

To use the Wait trait in the input pins, exti feature should be enabled for embassy-stme32, or gpiote feature should be enabled for embassy-nrf. That means, the input pins for stm32 could not share the same pin number(see the doc of ExtiInput)

Power consumption

~200ua when advertising

~450ua when idle (no key).

3ma when scanning and sending BLE keyboard report

TODO list

• basic implementation of async matrix
• test embassy-nrf, report the power consumption
• update rmk-macro to generate correct input pin types, add an option in keyboard.toml
• update documentation and examples
• update rmk-template