Expose onPointerDown handlers in <Button /> component

[Wriklend]

Currently, the Button component in TelegramUI uses a Tappable under the hood, which relies on onPointerDown and onPointerCancel for the ripple effect via the useRipple hook.

But since the Button doesn't explicitly support onPointerDown and onPointerUp props, there's no clean way to hook into these events from the outside. And even if you try to pass them manually through restProps, they end up replacing the internal handlers — which breaks the ripple effect.