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/examples/cln-plugin-startup.rs

@@ -6,7 +6,9 @@ use cln_plugin::{options, Builder, Error, Plugin};
 use tokio;
 #[tokio::main]
 async fn main() -> Result<(), anyhow::Error> {
-    if let Some(plugin) = Builder::new((), tokio::io::stdin(), tokio::io::stdout())
+    let state = ();
+
+    if let Some(plugin) = Builder::new(tokio::io::stdin(), tokio::io::stdout())
         .option(options::ConfigOption::new(
             "test-option",
             options::Value::Integer(42),
@@ -15,7 +17,7 @@ async fn main() -> Result<(), anyhow::Error> {
         .rpcmethod("testmethod", "This is a test", testmethod)
         .subscribe("connect", connect_handler)
         .hook("peer_connected", peer_connected_handler)
-        .start()
+        .start(state)
         .await?
     {
         plugin.join().await