title: TableAttribute is missing when change target from netcoreapp3.0 to netstandard2.1
body: <p>I figured I should move my class libraries to use .net standard 2.1.</p>
  
  <p>Is table attribute not in .net standard 2.1 ?  </p>
  
  <pre><code>using PWDRS.Core.Entities.Base;
  using System.Collections.Generic;
  using System.ComponentModel.DataAnnotations;
  using System.ComponentModel.DataAnnotations.Schema;
  
  namespace PWDRS.Core.Entities
  {
      [Table("ReportType", Schema = "RF")]
      public class RfReportType : EntityBase
      {
          public RfReportType()
          {
          }
  </code></pre>
  
  <blockquote>
    <p>The type or namespace name 'DataAnnotations' does not exist in the
    namespace 'System.ComponentModel' (are you missing an assembly
    reference?)</p>
    
    <p>The type or namespace name 'TableAttribute' could not be found (are
    you missing a using directive or an assembly reference?)</p>
    
    <p>The type or namespace name 'Table' could not be found (are you missing
    a using directive or an assembly reference?)</p>
    
    <p>The type or namespace name 'Schema' could not be found (are you
    missing a using directive or an assembly reference?)</p>
  </blockquote>