feat: Unify FlatLayout with other layouts and add metadata to their flatbuffer representation

vortex-flatbuffers/flatbuffers/vortex-serde/footer.fbs

@@ -1,24 +1,18 @@
-table FlatLayout {
-    begin: uint64;
-    end: uint64;
-}
-
-table NestedLayout {
+table Layout {
     children: [Layout];
     encoding: uint16;
-}
-
-union LayoutVariant {
-    FlatLayout,
-    NestedLayout,
-}
-
-table Layout {
-    layout: LayoutVariant;
+    metadata: [ubyte];
 }
 
 table Footer {
     layout: Layout;
+    row_count: uint64;
+}
+
+table Postscript {
+    schema_offset: uint64;
+    footer_offset: uint64;
 }
 
-root_type Footer;
+root_type Postscript;
+root_type Footer;