Add extension methods for reloadable options

[rynowak]

We want to add a set of extension methods for linking common patterns of use for reloadable options:

public static void Subscribe(this IFileProvider fileProvider, string filename,  IConfigurationRoot configuration)
{
    ChangeToken.OnChange((f) => f.Watch(filename), () => configuration.Reload());
}

public static void Subscribe<TOptions>(this IConfigurationRoot configuration, IReloadOptionsManager reload)
{
    ChangeToken.OnChange((c) => c.GetReloadToken(), () => reload.Reload<TOptions>());
}

These may live here, or in another repo.

[HaoK]

Added to Common in Primatives in ChangeToken static class