cln-grpc: Add midstate between configuration and replying to init

This is a bit special, in that it allows us to configure the plugin,
but then still abort startup by sending `init` with the `disable` flag
set.

plugins/src/messages.rs

@@ -132,6 +132,9 @@ pub(crate) struct GetManifestResponse {
 }
 
 #[derive(Serialize, Default, Debug)]
-pub struct InitResponse {}
+pub struct InitResponse {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub disable: Option<String>,
+}
 
 pub trait Response: Serialize + Debug {}