Add a `user_payment_id` to `get_payment_secret`+`PaymentReceived`

Add a `user_payment_id` to `get_payment_secret`+`PaymentReceived`

This allows users to store metadata about an invoice at
invoice-generation time and then index into that storage with a
general-purpose id when they call `get_payment_secret`. They will
then be provided the same index when the payment has been received.