title: react DataGrid: append custom filter operator without erasing the other operators
body: <p>I'm using <strong>MaterialUI's DataGrid</strong> for React and I have created a custom filter operator for one of the columns.<br>
  I was able to add it to the column as mentioned in the <a href="https://material-ui.com/components/data-grid/filtering/" rel="nofollow noreferrer">documentation</a> and it's working fine, however, this removed the other default operators (contains, equal, etc...).<br>
  This is how I did it:<br></p>
  <pre><code>columns: GridColDef[] = [
      {
          field: 'myFieldName',
          headerName: 'Column name',
          filterOperators: [MyCustomOperator],// this is the custom operator I have created
      }
  ]
  </code></pre>
  <br>
  How to append my custom operator and keep other operators?