[CAL-5182] Apple pay doesn't work with embed - It needs allow="payment" it seems

[hariombalhara]

Let's add allow="payment" attribute to iframe that we create via embed and also ensure that it doesn't break any existing functionality like embed should be able to redirect to a custom booking success page(that can be enabled in Advanced setting of an event-type).

_{From SyncLinear.com | CAL-5182}