A Sheets Connector sample to show how to update an existing spreadsheet

[meteatamel]

The connector_sheets.workflows.yaml sample shows how to create a new spreadsheet and update it using the connector. But it doesn't show how to update an existing spreadsheet. I believe this is more complicated as it'll have to involve authenticating the workflow with the user account where the spreadsheet lives. However, I think this would be a common use case that we should have a sample for.