chore: add initial events implementation

[brianium]

This is the initial event system for dues. Fairly standard run of the mill observer pattern.

An example listener may do something with taxes:

class TaxListener extends BaseEventListener
{
    public function onBeforeCreateSubscription(Subscription $subscription)
    {
        $tax = $this->calculateTax();
        $subscription->addOn(new AddOn('tax-fun', 1, new Price($tax)));
    }
}

// wherever Dues is instantiated:
$dues->addListener(new TaxListener());

A listener just has to implement the EventListener interface. A BaseEventListener class is provided to provide a stub implementation for all events.

[sonarcloud]

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