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

@@ -264,7 +264,7 @@ impl ConsoleLayer {
         // depending on the build-time configuration...
         #![allow(clippy::assertions_on_constants)]
         assert!(
-            cfg!(tokio_unstable),
+            cfg!(any(tokio_unstable, console_without_tokio_unstable)),
             "task tracing requires Tokio to be built with RUSTFLAGS=\"--cfg tokio_unstable\"!"
         );