cln-plugin: Add notification subscriptions and hooks to the plugins

For now hooks are treated identically to rpcmethods, with the
exception of not being returned in the `getmanifest` call. Later on we
can add typed handlers as well.

plugins/examples/cln-plugin-startup.rs

@@ -14,6 +14,8 @@ async fn main() -> Result<(), anyhow::Error> {
             "a test-option with default 42",
         ))
         .rpcmethod("testmethod", "This is a test", Box::new(testmethod))
+        .subscribe("connect", Box::new(connect_handler))
+        .hook("peer_connected", Box::new(peer_connected_handler))
         .start()
         .await?;
     plugin.join().await
@@ -22,3 +24,13 @@ async fn main() -> Result<(), anyhow::Error> {
 fn testmethod(_p: Plugin<()>, _v: &serde_json::Value) -> Result<serde_json::Value, Error> {
     Ok(json!("Hello"))
 }
+
+fn connect_handler(_p: Plugin<()>, v: &serde_json::Value) -> Result<(), Error> {
+    log::info!("Got a connect notification: {}", v);
+    Ok(())
+}
+
+fn peer_connected_handler(_p: Plugin<()>, v: &serde_json::Value) -> Result<serde_json::Value, Error> {
+    log::info!("Got a connect hook call: {}", v);
+    Ok(json!({"result": "continue"}))
+}