cln-plugin: make available the configuration in plugin

There are several cases where you want to access to the configuration,
and given the nature of the rust API, there is no way to access to
the `configuration` field at any point after the configuration logic.

Suggested-by: Sergi Delgado Segura <@sr-gi>

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

plugins/src/lib.rs

@@ -495,6 +495,12 @@ where
             .next()
             .map(|co| co.value.clone().unwrap_or(co.default().clone()))
     }
+
+    /// return the cln configuration send to the
+    /// plugin after the initialization.
+    pub fn configuration(&self) -> Configuration {
+        self.configuration.clone()
+    }
 }
 
 impl<S> PluginDriver<S>