Define payload property in Event class

Summary:
This allows to use code like:

function handleEvent (Event $event): void {
    $this->doStuff($event->payload); // This is now always valid.
}

Test Plan: Run tests, then phan | grep payload.

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: alken-orin

Differential Revision: https://devcentral.nasqueron.org/D781