partial stub out of decoder

src/common/cpp_essentials/structured_append.rs

@@ -0,0 +1,16 @@
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct StructuredAppendInfo {
+    pub index: i32, // -1;
+    pub count: i32, // = -1;
+    pub id: String,
+}
+
+impl Default for StructuredAppendInfo {
+    fn default() -> Self {
+        Self {
+            index: -1,
+            count: -1,
+            id: Default::default(),
+        }
+    }
+}