title: filter for custom column in grid view which is not in table yii2
body: <p>I have added this 'sla_status' field in gridview and every thing is working fine only issue is I am unable to set filter values for this.
  'sla_status' is not in my table.</p>
  
  <pre><code>[
              'label' =&gt; Yii::t('app','Sla Status'),
              'format' =&gt; 'raw',
              'filterType' =&gt; GridView::FILTER_SELECT2,
              'filter' =&gt; $status,
              'filterWidgetOptions' =&gt; [ 
                      'options' =&gt; [ 
                              'placeholder' =&gt; Yii::t('app','All...' )
                      ],
                      'pluginOptions' =&gt; [ 
                              'allowClear' =&gt; true 
                      ] 
              ],
  
              'headerOptions' =&gt; ['style' =&gt; 'text-align:center;color:#337ab7'],
              'value' =&gt; function ($model, $key, $index, $widget)
              {           }   
              ]
  </code></pre>
  
  <p>and $status is <code>$status = array('0'=&gt;Yii::t('app', 'Inactive'),'1'=&gt;Yii::t('app', 'Active'));</code></p>