Update changelog based on model updates

Author: aws-sdk-python-automation

.changes/next-release/api-change-appconfig-67113.json

@@ -0,0 +1,5 @@
+{
+  "type": "api-change",
+  "category": "``appconfig``",
+  "description": "Adding Create, Get, Update, Delete, and List APIs for new two new resources: Extensions and ExtensionAssociations."
+}