Add schema into_builder

crates/iceberg/src/spec/schema.rs

@@ -269,6 +269,16 @@ impl Schema {
         }
     }
 
+    /// Create a new schema builder from a schema.
+    pub fn into_builder(self) -> SchemaBuilder {
+        SchemaBuilder {
+            schema_id: self.schema_id,
+            fields: self.r#struct.fields().iter().cloned().collect(),
+            alias_to_id: self.alias_to_id,
+            identifier_field_ids: self.identifier_field_ids,
+        }
+    }
+
     /// Get field by field id.
     pub fn field_by_id(&self, field_id: i32) -> Option<&NestedFieldRef> {
         self.id_to_field.get(&field_id)