Add a way to check whether the context is being shut down

[Keruspe]

Is your feature request related to a problem? Please describe.
From within a library, I want to detect whether the tokio context from the calling application is being shutdown to properly clean things up instead of emitting undesired errors. Right now, I'm getting: Error { kind: IOError(Custom { kind: Other, error: "A Tokio 1.x context was found, but it is being shutdown." } which I then forward to the calling application.

Describe the solution you'd like
Something like handle.is_shutting_down() or even a basic tokio::is_shutdown_error(err) would be perfect to be able to handle this scenario.

Describe alternatives you've considered
Alternative is letting the application handle this, but this is only moving the problem around, I think.
I could also match on the error message, but it's not exported by tokio and might as well change at some point

[Darksonn]

Some mechanism to check whether an IO error is a shutting down runtime seems reasonable. It would be nice to simplify this internal logic through the same mechanism:

tokio/tokio/src/process/unix/pidfd_reaper.rs

Lines 127 to 135 in f490029

	fn is_rt_shutdown_err(err: &io::Error) -> bool {
	if let Some(inner) = err.get_ref() {
	err.kind() == io::ErrorKind::Other
	&& inner.source().is_none()
	&& display_eq(inner, RUNTIME_SHUTTING_DOWN_ERROR)
	} else {
	false
	}
	}