Add dropin presentation info to completion callbacks

[mtroskot]

Is your feature request related to a problem? Please describe. When using AdyenDropIn, there's no way to know if the dropin UI was actually presented to the user. This makes it impossible to track payment flow analytics, debug issues, or optimize UX based on whether users saw the payment method selection screen.

Describe the solution you'd like

Add dropin presentation information through one of these approaches:

Option 1: New callback

<AdyenCheckout 
  onComplete={onComplete}
  onError={onError}
  onDropinPresented={() => {
    // Track that dropin UI was shown
  }}
/>

Option 2: Flag in useAdyenCheckout hook

const { start, dropinPresented } = useAdyenCheckout();

// dropinPresented becomes true when dropin UI is displayed
useEffect(() => {
  if (dropinPresented) {
    // Track dropin presentation
  }
}, [dropinPresented]);

Either approach would allow tracking whether the dropin UI was actually presented during the payment flow, enabling better analytics and debugging capabilities.

Describe alternatives you've considered

• Manual state tracking (error-prone)
• Custom event listeners (adds complexity)
• Configuration inference (doesn't reflect runtime behavior)

Additional context Needed for payment flow analytics, A/B testing, and customer support in our MOTO payment implementation. Should be backward compatible with default false value.

[descorp]

Thanks for interesting use case @mtroskot

We will discuss internally with the product