examples/rp add multicore_multiprio example

[orukusaki]

Adds an example to demonstrate using the new Multicore Executors added in #5409

[sourcebox]

As all examples share the same Cargo.toml either all of them have to be updated or this one has to go into a separate subdirectory so it can have a separate configuration.

The main question to be answered is probably whether the user should always use the custom executors to avoid any possible issues.

[orukusaki]

I see what you mean. Another approach might be to copy what was done for mcxa5xx/src/bin/power-deepsleep-big-jump.rs etc and put cfg flags in the example

[sourcebox]

When we discussed it on Matrix, @lulf suggested using a subdirectory, @jamesmunns pointed me to the MCXA example with the cfg flags.

@Dirbaio: what's your opinion on this?