Missing canceled status for admin/staff initiated cancellations

[YounesOMK]

no way to cancel payments from admin when they're stuck in waiting/input

have payments sitting there for days when users abandon checkout.I don't think we can't just use rejected since that's for gateway rejections, not manual cancellation

[WhyNotHugo]

We're missing:

• PaymentStatus.Cancelled
• Handling of cancellation events in notifications.
• A method to cancel a Payment instance.
• An action in the admin to cancel payments.

I believe we can handle these three tasks as individual steps. We can simply raise NotImplementedError via the base provider and gradually implement cancellation for each provider.