Example using Microsoft.Azure.Management.WebSites causes NuGet warning

We get the red triangle of deprecation in the NuGet package manager because the `Corvus.Identity.Examples.UsingMicrosoftRest` project uses `Microsoft.Azure.Management.Websites`.

NuGet is telling us to upgrade to `Azure.ResourceManager.AppService` but that would be the wrong thing to do. The entire point of this example is to illustrate how to use the support for the old-school `Microsoft.Rest` authentication mechanisms. If we upgraded to the new management NuGet package, we'd need to move to the `Azure.Core`/`Azure.Identity` mechanisms, and we already have an example illustrating that: `Corvus.Identity.Examples.UsingAzureCore`

So what we might need to do is pick some other way of demonstrating how to use `Microsoft.Rest`-based auth. This might entail avoiding Azure client SDKs entirely, because Microsoft is gradually deprecating all of those as they fill in the gaps with `Azure.Core`-based versions.