Make IConfiguration dependency an optional parameter in the AddApplicationInsightsTelemetry method

[olegsych]

... because it requires adding non-trivial code to a WebAPI project, which, out of the box looks like this:

public class Startup
{
    public Startup(IHostingEnvironment env) {
    }
    public void ConfigureServices(IServiceCollection services) {
        services.AddMvc();
    }
    public void Configure(IApplicationBuilder app, IHostingEnvironment env) {
        app.UseStaticFiles();
        app.UseMvc();
    }
}

[dnduffy]

This has been done.