cln_plugin: Example package subscribing to "*"

Creates an example package that subscribes to all notifications and logs
them. This is useful for testing the behavior of subscribing to "*".

I've also edited the Makefile to ensure that `make` builds the example
and that `make clean` removes the example

plugins/examples/cln-subscribe-wildcard.rs

@@ -0,0 +1,35 @@
+/// This plug-in subscribes to the wildcard-notifications
+/// and creates a corresponding log-entry
+
+use anyhow::Result;
+use cln_plugin::{Builder, Plugin};
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    let state = ();
+
+    let configured = Builder::new(tokio::io::stdin(), tokio::io::stdout())
+        .subscribe("*", handle_wildcard_notification)
+        .start(state)
+        .await?;
+
+    match configured {
+	Some(p) => p.join().await?,
+	None => return Ok(()) // cln was started with --help
+    };
+
+    Ok(())
+}
+
+async fn handle_wildcard_notification(_plugin: Plugin<()>, value : serde_json::Value) -> Result<()> {
+    let notification_type : String = value
+	.as_object()
+	.unwrap()
+	.keys()
+	.next()
+	.unwrap()
+	.into();
+
+    log::info!("Received notification {}", notification_type);
+    Ok(())
+}