[FR] Messaging: Support for iOS 18.4 Declarative push messages

Declarative push messages are a new feature in iOS 18.4 that allow push messages to web apps without service worker handling. This greatly improves the deliverability of background notifications for web apps on iOS.

This requires the addition of an extra top level field in the Message interface as described [here](https://pr-preview.s3.amazonaws.com/w3c/push-api/pull/385.html#declarative-push-message).

Please allow the optional "web_push" (integer) field to the Message interface to enable Declarative push messages.

Currently, when adding this field, I receive the following validation error:

Error: Failed to send notification: Invalid JSON payload received. Unknown name "web_push" at 'message.webpush': Cannot find field.

More information on this feature [here](https://webkit.org/blog/16535/meet-declarative-web-push/#how-to-use-declarative-web-push).