Add support for supplying refresh token for OAuth source in Gateways - Update Datasource API

We are connecting to our data source using OAuth credentials. Sometimes, Power BI requires us to re-enter our credentials in the Service, when the token has been invalidated for some reason (e.g. change of dataset ownership). We then update the OAuth credentials via the "Gateways - Update Datasource" API. We provide a valid Bearer Token as per the OAuth update example and everything initially works as expected. However, obviously the bearer token expires after an hour (the default for most bearer tokens), and the Power BI Service asks us to re-enter our credentials. We then have to run this code every hour (or at least before any refresh) to make sure the credentials don't expire. This doesn't happen if you update the credentials through the UI - refreshing of the token must be being handled by the Power BI Service behind the scenes. But there is no option to provide a "refresh token" in the above "Update Datasource" API. Please add support for supplying a refresh token when updating datasource credentials programmatically for an OAuth-authenticated data source.