External Tools: provide more % arguments to send to an external tool

Currently the only available arguments are %server% and %database% to connect an external tool to the tabular model. Add at least %pbixfile% argument (with a full path to an active pbixfile). If possible also add %issaved% argument ('true' if active file is saved, 'false' if there are unsaved changes).

This will allow to use external tools to back up .pbix files and to publish (and do other manipulations in Power BI Service) reports using custom PowerShell scripts and REST API.