Update Credentials of Dataset using API

It is imperative to have parity between the UI controls in the Power BI web interface and the REST API.

One clear example of this is the ability to edit/update data source credentials. The API currently grants the ability to update the connection string of a data source, but not the credentials. Additionally, we're able to edit the credentials of a data gateway and the associated data sources, but not the full range of data sources available to Power BI (e.g. Azure SQL, SQL Server, etc).

This will allow the deployment of .PBIX files to be fully automated (aside from some other suggestions that request parameters to be added to the API, which is also a great idea). Right now, I'm able to import a .PBIX file through the API, but still require some sort of user intervention in the web interface to update the data source credentials. For more sensitive datasets, this can't fly - we're trying to obfuscate the credentials from the users by building an application to manage the deployment for them.