Add RTT input support.

[Dirbaio]

This PR adds RTT input support on down channel 0. It is enabled with flag --input (not on by default).

Since defmt-rtt doesn't expose RTT, this can be used with a custom defmt logger implementation such as this one (example).

Missing things:

• To read from stdin in the main loop, it is switched to nonblocking mode. This currently only works on unixes. Don't know if there's a Windows equivalent or we should read stdin from a separate thread.
• Input is only sent after pressing Return. We could switch to raw mode so every keystroke is sent, but IMO it should be optional since the host terminal's line editing is useful.