chore: create ResourceNotificationMessage and AsNotifier (#15301)

Closes https://github.com/coder/coder/issues/15213

This PR enables sending notifications without requiring the auth system
context, instead using a new auth notifier context.

docs/reference/api/schemas.md

@@ -4491,6 +4491,7 @@ CreateWorkspaceRequest provides options for creating a new workspace. Only one o
 | `group_member`            |
 | `idpsync_settings`        |
 | `license`                 |
+| `notification_message`    |
 | `notification_preference` |
 | `notification_template`   |
 | `oauth2_app`              |