cln_plugin: add shutdown() method to Plugin

When plugins receive a "shutdown" notification, then can call this
method which will shutdown `cln_plugin`.

Then they can await `plugin.join()` and do any remaining cleanup
there.

This helps avoid a pain-point where plugin authors need to handle
2 separate plugin shutdown mechanisms https://github.com/ElementsProject/lightning/issues/6040

plugins/src/lib.rs

@@ -691,6 +691,7 @@ impl<S> Plugin<S>
 where
     S: Send + Clone,
 {
+    /// Wait for plugin shutdown
     pub async fn join(&self) -> Result<(), Error> {
         self.wait_handle
             .subscribe()
@@ -698,6 +699,14 @@ where
             .await
             .context("error waiting for shutdown")
     }
+
+    /// Request plugin shutdown
+    pub fn shutdown(&self) -> Result<(), Error> {
+        self.wait_handle
+            .send(())
+            .context("error waiting for shutdown")?;
+        Ok(())
+    }
 }
 
 #[cfg(test)]