Allow event-sourced aggregates to specify how to initialize themselves from first event

[subhashb]

The first event generated by an event-sourced aggregate is used to initialize the aggregate instance. The remaining events are applied through methods marked apply.

        # Initialize the aggregate with the first event's payload and apply it
        aggregate = cls(**events[0].payload)
        aggregate._apply(events[0])

        # Apply the rest of the events
        for event in events[1:]:
            aggregate._apply(event)

However, this only works if the first event mirrors the aggregate's attribute structure.

We need to provide a way for aggregates to specify a special method to initialize itself from the first event.