Repeat on hold button behavior

[ARoese]

This PR adds a repeat-on-hold feature. It is enabled by setting the new held_repeat_interval config field to Some(Duration) and is disabled otherwise. When the button is held down for the long_press duration, it emits a ButtonEvent::LongPress as usual. Then, for the duration the button continues to be held, a ButtonEvent::ShortPress{count: 1} will be emitted every held_repeat_interval until it is released.

This functionality is useful for settings buttons, as it allows a user to press and hold an up or down button instead of needing to rapidly mash and wear it out. Surely, there are also other applications.

This is an API-breaking change, since a field is added to ButtonConfig. It also adds a dependency on embassy-futures for use embassy_futures::select::{select, Either}. Since there is already an embassy-time dependency, I figure that another well-known crate from the same ecosystem is fine.

I also added a unit test for the feature.