Allow hooking into the executor

[bugadani]

This PR should enable both #3728 and #3670 without actually integrating them into the executor.

[bjoernQ]

I guess for s.th. like #3728 embassy-executor's rtos-trace feature (or embassy-rs/embassy#4033) would be a better fit.

While the above might also work for #3670 this "explicit" callback approach feels better suited there

[bugadani]

Yeah the only advantage of this approach is that this is free for executors that don't opt into it (while tracing has a runtime cost to check if the right executor is called), and it's not called "tracing" when you want to inject an esp-wifi yield :)

[bjoernQ]

Code looks good - just wondering if it's worth to add a test for this

[bugadani]

What would you like to be tested?

[bjoernQ]

What would you like to be tested?

Nothing fancy - just something exercising run_with_callbacks making sure it works the same way as run when the callbacks do nothing - as said I'm not sure it's worth it - approving and leaving that up to you