Make InAppMessage.customPayload optional in type definition

The customPayload property can be undefined when no RAW JSON is
configured in the Iterable dashboard. Update the type to accurately
reflect runtime behavior.

Fixes #538

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: franco-zalamena-iterable

src/inapp/types.ts

@@ -50,7 +50,7 @@ export interface InAppMessage {
     };
     webInAppDisplaySettings: WebInAppDisplaySettings;
   };
-  customPayload: Record<string, any>;
+  customPayload?: Record<string, any>;
   trigger: { type: string };
   saveToInbox: boolean;
   inboxMetadata: {