[Feature] Add debug-keys support

[wipawel]

Is your feature request related to a problem? Please describe.
When UART input or PS/2 keyboard support is added, we can add debug keys support for various runtime actions.

Describe the solution you'd like
I would implement a register_debugkey(key, callback, help_msg) interface to assign callback functions to dedicated key combinations.

I would support the following keys:

• '?' - list all registered keys, functions and help msgs
• 'p' - pause execution of BSP (or all CPUs - that would require IPI support)
• 'u' - unpause the above
• 'r' - dump register content
• 't' - dump page tables content
• 'c' - crash
• '+' - toggle debug mode