cln-plugin: Add support for synchronous RPC methods

Changelog-Experimental: cln-plugin: Added support for non-async RPC method passthrough (async support coming soon)

plugins/examples/cln-plugin-startup.rs

@@ -1,9 +1,10 @@
 //! This is a test plugin used to verify that we can compile and run
 //! plugins using the Rust API against c-lightning.
-
-use cln_plugin::{options, Builder};
+#[macro_use]
+extern crate serde_json;
+use cln_plugin::{options, Builder, Error, Plugin};
+use std::pin::Pin;
 use tokio;
-
 #[tokio::main]
 async fn main() -> Result<(), anyhow::Error> {
     let plugin = Builder::new((), tokio::io::stdin(), tokio::io::stdout())
@@ -12,7 +13,12 @@ async fn main() -> Result<(), anyhow::Error> {
             options::Value::Integer(42),
             "a test-option with default 42",
         ))
+        .rpcmethod("testmethod", "This is a test", Box::new(testmethod))
         .start()
         .await?;
     plugin.join().await
 }
+
+fn testmethod(_p: Plugin<()>, _v: &serde_json::Value) -> Result<serde_json::Value, Error> {
+    Ok(json!("Hello"))
+}