Change ConfigurationRoot to Configuration in AppSettingsProvider

[CumpsD]

In .NET Core 2.0 it is advised to switch to IConfiguration: https://docs.microsoft.com/en-us/aspnet/core/migration/1x-to-2x/?view=aspnetcore-2.0#add-configuration-providers

This means there is no IConfigurationRoot anymore to pass to AppSettingsProvider https://github.com/jason-roberts/FeatureToggle/blob/master/src/FeatureToggle.Shared/Internal/AppSettingsProvider.cs#L32

[jason-roberts]

Thanks @CumpsD - we'll look at this in v5