samples: modules: lvgl: add keypad and encoder demo

[everedero]

Adds the LVGL keypad and encoder demo to sample build.

[kartben]

Nice! Didn't know about this sample :)
@faxe1008 it almost feels like we should really get the "basic" sample back to a minimal one like we used to have if demos allows folks to quickly try out more complex scenarios, inc. input, on their boards? But then there's also things like #67508 that can be quite useful ...

[faxe1008]

@everedero thanks for your submission, indeed this sample was missing :^). Overall looks fine to me, but I was wondering if an overlay might make sense here because this sample does not work "out of the box" with the native_sim as the sample readme would suggest. Using zephyr,native-linux-evdev could be an elegant solution for this.

@kartben agree on this. After this PR is merged pretty much all indev are covered in other samples (except the button indev one) and could be removed, making the sample more basic once again 👍

[everedero]

@faxe1008 That would be nice indeed, I will try to add this.

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[everedero]

Will be done in PR #68168