Fallback from WAM to browser on B2C, extend Webview2 support

[bgavrilMS]

Fixes #4072

Changes proposed in this request

• Disallow WAM on non-AAD (e.g. B2C)
• Allow webview2 on ADFS
• New API for adding WV2 support from MSAL.Desktop

Testing

Performance impact

Documentation

• All relevant documentation is updated.

[pmaytak]

Also should update this error message to

                throw new MsalClientException(MsalError.WebviewUnavailable,
                    "If you have a Windows application which targets net5.0 or above, change the target to at least net6.0-windows, " +
                        "which provides support from Windows 7 to Windows 10. For details, see https://github.com/dotnet/designs/blob/main/accepted/2020/platform-checks/platform-checks.md" +

                    "If you have a cross-platform (Windows, Mac, Linux) application which targets net6.0, dual target net6.0 and net6.0-windows." +
                        "Your installer should deploy the net6.0 version on Mac and Linux and the net6.0-window on Windows." +
                        "For details, see https://github.com/dotnet/designs/blob/main/accepted/2020/platform-checks/platform-checks.md" +

                    "If you have a .NET Core 3.1 app, reference the NuGet package Microsoft.Identity.Client.Desktop and call the extension method .AddEmbeddedBrowserSupport() first." + "For details, see https://aka.ms/msal-net-webview2 or use the system WebView - see https://aka.ms/msal-net-os-browser");