Creating extensions for ZAP files using .zapExtension files (#1552)

- .zapExtension files can now be used to extend the .zap files to add additional clusters, commands or attributes on an endoint
- Adding the logic to do this during generation or during file open using command line arguments.
- Adding unit tests
- Adding documentation on usage
- Added an attribute to the xml so updated one of the existing tests
- Fixing existing tests and adding tests for zap extensions with clusters and attributes on an endpoint type id
- Linking the extension file to the endpoint identifiers and not endpoint type ids
- Github: ZAP#1254