public static IConfiguration Configuration { get; private set; }
public Startup(IConfiguration configuration)
{
    Configuration = configuration;
}
public void ConfigureServices(IServiceCollection services)
{
    //添加服务
    services.AddUrlResolve(Configuration);
}

/// <summary>
/// 拓展方法
/// </summary>
public static class Extensions
{
    /// <summary>
    /// 为了保留IConfiguration
    /// </summary>
    /// <param name="services"></param>
    /// <param name="config"></param>
    public static void AddUrlResolve(this IServiceCollection services, IConfiguration config)
    {
        services.Add(new ServiceDescriptor(typeof(UrlResolveService), typeof(UrlResolveService), ServiceLifetime.Singleton));
        //读取配置文件
        UrlResolveService.ApiUrl = config["ApiUrl"];
        UrlResolveService.AuthorityUrl = config["AuthorityUrl"];
    }
}

/// <summary>
/// 读取配置文件的服务
/// </summary>
public class UrlResolveService
{
    public static string AuthorityUrl { get; set; }
    public static string ApiUrl { get; set; }

    public string GetAuthorityUrl()
    {
        return AuthorityUrl;
    }
    public string GetApiUrl()
    {
        return ApiUrl;
    }
}

public async Task<IActionResult> Call(UrlResolveService urlResolveService)
{
    //API对应网址
    var apiUrl = urlResolveService.GetApiUrl();
    //......
}

Microsoft.Extensions.Configuration
Microsoft.Extensions.Configuration.FileExtensions
Microsoft.Extensions.Configuration.Json

var builder = new ConfigurationBuilder()
			//保证执行文件下有json文件
			.SetBasePath(Directory.GetCurrentDirectory())
			.AddJsonFile("appsettings.json");
IConfiguration config = new ConfigurationBuilder()
	.AddJsonFile("appsettings.json", true, true)
	.Build();
var yourValue = config["YourKey"];