feat(subscriber): Add cfg console_without_tokio_unstable

Add cfg `console_without_tokio_unstable` for developers to turn off the
assertion on `tokio_unstable`. This is useful for non-tokio runtimes
which has `tokio-console` support.

console-subscriber/src/lib.rs

@@ -263,6 +263,7 @@ impl ConsoleLayer {
         // The `cfg` value *appears* to be a constant to clippy, but it changes
         // depending on the build-time configuration...
         #![allow(clippy::assertions_on_constants)]
+        #[cfg(not(console_without_tokio_unstable))]
         assert!(
             cfg!(tokio_unstable),
             "task tracing requires Tokio to be built with RUSTFLAGS=\"--cfg tokio_unstable\"!"