psbt: Add "fee" field accessor

src/stdlib/psbt.rs

@@ -749,6 +749,12 @@ impl FieldAccess for Psbt {
             "xpub" => self.xpub.into(),
             "unknown" => self.unknown.into(),
             "proprietary" => self.proprietary.into(),
+            "fee" => {
+                let fee = self.fee().ok();
+                fee.and_then(|f| f.to_sat().try_into().ok())
+                    .unwrap_or(-1i64)
+                    .into()
+            }
             _ => {
                 return None;
             }