fix: Make signal in EventPublisherSubscribeIteratorOptions AbortSignal | undefined (#928)

If it isn't, while using the `signal` prop from the handler function, my
typescript complains with `exactOptionalPropertyTypes` enabled:

```
Argument of type '{ signal: AbortSignal | undefined; }' is not assignable to parameter of type 'EventPublisherSubscribeIteratorOptions' with 'exactOptionalPropertyTypes: true'. Consider adding 'undefined' to the types of the target's properties.
      Types of property 'signal' are incompatible.
        Type 'AbortSignal | undefined' is not assignable to type 'AbortSignal'.
          Type 'undefined' is not assignable to type 'AbortSignal'.
```

Ref.: https://orpc.unnoq.com/docs/event-iterator#event-publisher

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Adjusted public API option to be more permissive with cancellation
signals, aligning types with existing behavior.
  * No changes to runtime behavior or user-facing functionality.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: boredland

packages/shared/src/event-publisher.ts

@@ -21,7 +21,7 @@ export interface EventPublisherSubscribeIteratorOptions extends EventPublisherOp
   /**
    * Aborts the async iterator. Throws if aborted before or during pulling.
    */
-  signal?: AbortSignal
+  signal?: AbortSignal | undefined
 }
 
 export class EventPublisher<T extends Record<PropertyKey, any>> {