feat: add support for custom client notifications (#556)

MCP servers, particularly ones that offer "experimental" capabilities,
may wish to handle custom client notifications that are not part of the
standard MCP specification. This change introduces a new
`CustomClientNotification` type that allows a server to process
such custom notifications.

- introduces `CustomClientNotification` to carry arbitrary methods/params while
  still preserving meta/extensions; wires it into the `ClientNotification` union
  and `serde` so `params` can be decoded with `params_as`
- allows server handlers to receive custom notifications via a new
  `on_custom_notification` hook
- adds integration coverage that sends a custom client notification end-to-end
  and asserts the server sees the method and payload

Test:

```shell
cargo test -p rmcp --features client test_custom_client_notification_reaches_server
```

crates/rmcp/src/handler/server.rs

@@ -90,6 +90,9 @@ impl<H: ServerHandler> Service<RoleServer> for H {
             ClientNotification::RootsListChangedNotification(_notification) => {
                 self.on_roots_list_changed(context).await
             }
+            ClientNotification::CustomClientNotification(notification) => {
+                self.on_custom_notification(notification, context).await
+            }
         };
         Ok(())
     }
@@ -225,6 +228,14 @@ pub trait ServerHandler: Sized + Send + Sync + 'static {
     ) -> impl Future<Output = ()> + Send + '_ {
         std::future::ready(())
     }
+    fn on_custom_notification(
+        &self,
+        notification: CustomClientNotification,
+        context: NotificationContext<RoleServer>,
+    ) -> impl Future<Output = ()> + Send + '_ {
+        let _ = (notification, context);
+        std::future::ready(())
+    }
 
     fn get_info(&self) -> ServerInfo {
         ServerInfo::default()