[Darc CLI] --what-if mode for darc add-subscription

[rainersigwald]

Many tools have a --dry-run or --what-if parameter that can be added to prevent destructive remote operations while presenting the user a view of what the tool would have done if run without the parameter.

I'm looking at setting up new subscriptions for the release branch in MSBuild while documenting how to do that for the future, and I'd love to see such an option on darc add-subscription.

[riarenas]

I like the idea and doesn't seem that expensive. Moving to improvements epic to re-examine after we're done with the .NET 5 business priorities.