要修改 GetSection 方法的内容，需要自定义一个 IConfiguration 实现类，并通过 builder.Services.AddSingleton 注册为服务。示例如下：

public class MyConfiguration : IConfiguration
{
    private IConfiguration _innerConfig;
    
    public MyConfiguration(IConfiguration config)
    {
        _innerConfig = config;
    }
    
    public IConfigurationSection GetSection(string key)
    {
        // your custom implementation
        return _innerConfig.GetSection(key);
    }

    public IEnumerable GetChildren()
    {
        return _innerConfig.GetChildren();
    }

    public IChangeToken GetReloadToken()
    {
        return _innerConfig.GetReloadToken();
    }

    public string this[string key]
    {
        get => _innerConfig[key];
        set => _innerConfig[key] = value;
    }
}

public void ConfigureServices(IServiceCollection services)
{
    // build your configuration
    var configurationBuilder = new ConfigurationBuilder();
    configurationBuilder.AddJsonFile("appsettings.json", optional: false);
    var config = configurationBuilder.Build();

    // register your custom configuration
    services.AddSingleton(new MyConfiguration(config));
}