feat: add Optional invocie to restore response

Author: i5hi

src/swaps/boltz.rs

@@ -996,6 +996,8 @@ pub struct SwapRestoreResponse {
     pub created_at: u64,
     pub from: String,
     pub to: String,
+    /// Lightning invoice; boltz only returns it for submarine/reverse swaps.
+    pub invoice: Option<String>,
     pub claim_details: Option<ClaimDetails>,
     pub refund_details: Option<RefundDetails>,
 }