Adding clusterExtension Support from standard xml file loading like we do from zcl.json (#1532)

* Adding clusterExtension Support from standard xml file loading like we do from zcl.json

- Updating the zcl loader to load struct items after it has loaded all the other data types across all xml files such that there are no undefined data types for struct items.
- Added Unit tests for the same
- updating existing unit tests because 1 command and 2 command args were added
- Github: ZAP#1529