Add an execute method for the Event Definitions

[caleeli]

The execute method implements the event definition behavior when an event is triggered.

    /**
     * Implement the event definition behavior when an event is triggered.
     *
     * @param EventDefinitionInterface $event
     * @param FlowNodeInterface $target
     * @param ExecutionInstanceInterface|null $instance
     * @param TokenInterface|null $token
     *
     * @return $this
     */
    public function execute(EventDefinitionInterface $event, FlowNodeInterface $target, ExecutionInstanceInterface $instance = null, TokenInterface $token = null);