cln-plugin: Defer binding the plugin state until after configuring

We had a bit of a chicken-and-egg problem, where we instantiated the
`state` to be managed by the `Plugin` during the very first step when
creating the `Builder`, but then the state might depend on the
configuration we only get later. This would force developers to add
placeholders in the form of `Option` into the state, when really
they'd never be none after configuring.

This defers the binding until after we get the configuration and
cleans up the semantics:

 - `Builder`: declare options, hooks, etc
 - `ConfiguredPlugin`: we have exchanged the handshake with
   `lightningd`, now we can construct the `state` accordingly
 - `Plugin`: Running instance of the plugin

Changelog-Changed: cln-plugin: Moved the state binding to the plugin until after the configuration step

plugins/grpc-plugin/src/main.rs

@@ -22,13 +22,7 @@ async fn main() -> Result<()> {
     let directory = std::env::current_dir()?;
     let (identity, ca_cert) = tls::init(&directory)?;
 
-    let state = PluginState {
-        rpc_path: path.into(),
-        identity,
-        ca_cert,
-    };
-
-    let plugin = match Builder::new(state.clone(), tokio::io::stdin(), tokio::io::stdout())
+    let plugin = match Builder::new(tokio::io::stdin(), tokio::io::stdout())
         .option(options::ConfigOption::new(
             "grpc-port",
             options::Value::Integer(-1),
@@ -54,7 +48,13 @@ async fn main() -> Result<()> {
         Some(o) => return Err(anyhow!("grpc-port is not a valid integer: {:?}", o)),
     };
 
-    let plugin = plugin.start().await?;
+    let state = PluginState {
+        rpc_path: path.into(),
+        identity,
+        ca_cert,
+    };
+
+    let plugin = plugin.start(state.clone()).await?;
 
     let bind_addr: SocketAddr = format!("0.0.0.0:{}", bind_port).parse().unwrap();