multi: add blinding point to payment descriptor and persist

This commit adds an optional blinding point to payment descriptors and
persists them in our HTLC's extra data. A get/set pattern is used to
populate the ExtraData on our disk representation of the HTLC so that
callers do not need to worry about the underlying storage detail.

lnwire/update_add_htlc.go

@@ -78,6 +78,19 @@ type UpdateAddHTLC struct {
 	ExtraData ExtraOpaqueData
 }
 
+// BlingingPointOrNil returns the blinding point associated with the update, or
+// nil.
+func (c *UpdateAddHTLC) BlingingPointOrNil() *btcec.PublicKey {
+	var blindingPoint *btcec.PublicKey
+	c.BlindingPoint.WhenSome(func(b tlv.RecordT[BlindingPointTlvType,
+		*btcec.PublicKey]) {
+
+		blindingPoint = b.Val
+	})
+
+	return blindingPoint
+}
+
 // NewUpdateAddHTLC returns a new empty UpdateAddHTLC message.
 func NewUpdateAddHTLC() *UpdateAddHTLC {
 	return &UpdateAddHTLC{}