core 获取 appsettings.json

 var builder = new ConfigurationBuilder()
                  .SetBasePath(Directory.GetCurrentDirectory())
                     .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true);

        IConfigurationRoot configuration = builder.Build();
        var mysqlConn = configuration.GetSection("ConnectionStrings:Mysql").Value;
        var sqlserverConn = configuration.GetSection("ConnectionStrings:SqlServer").Value;