(scheduler-targets-alpha): Make the event bus in EventBridgePutEventsEntry optional

### Describe the feature

When creating a `EventBridgePutEvents` target for a `Schedule` the event entry has to be of type `EventBridgePutEventsEntry`. The shape of the interface looks like this:
```
export interface EventBridgePutEventsEntry {
readonly detail: ScheduleTargetInput;
readonly detailType: string;
readonly eventBus: events.IEventBus;
readonly source: string;
}
```

For me as an user it would be good to have the `eventBus` as an optional property. So when the eventBus is not defined, the default event bus is used.

### Use Case

Creating a target without specifying an event bus (just use the default one).

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.141.0

### Environment details (OS name and version, etc.)

Mac

Start at the EventBridgePutEventsEntry interface and the EventBridgePutEvents scheduler target entry point. Trace how an omitted eventBus is handled when creating the target, then verify that the default EventBridge bus is used while existing explicit-bus behavior remains unchanged.

Written by the indexing model from the issue text.