Make `RFQ.PrivateData` generic

[jiyoonie9]

something like this, so we can pass in the types in scrubbedRfq, privateData, err := rfq.Scrub(PayinType, PayoutType) and get back strongly typed privatedata in the returned privateData struct

type PrivateData[I, O] struct {
	Salt   string                `json:"salt,omitempty"`
	Claims []string              `json:"claims,omitempty"`
	Payin  PrivatePaymentDetails `json:"payin,omitempty"`
	Payout PrivatePaymentDetails `json:"payout,omitempty"`
}```