feat(explore): make dnd controls clickable (apache#16119)

* Make ghost buttons clickable

* Popover for column control

* Make column dnd ghost button clickable

* Prefill operator only if column is defined

* Remove data-tests

* lint fix

* Hide new features behind a feature flag

* Change ghost button texts

* Remove caret for non clickable columns

(cherry picked from commit 203c311)

superset-frontend/src/explore/components/controls/DndColumnSelectControl/DndColumnSelect.tsx

@@ -16,8 +16,8 @@
  * specific language governing permissions and limitations
  * under the License.
  */
-import React, { useCallback, useMemo } from 'react';
-import { tn } from '@superset-ui/core';
+import React, { useCallback, useMemo, useState } from 'react';
+import { FeatureFlag, isFeatureEnabled, tn } from '@superset-ui/core';
 import { ColumnMeta } from '@superset-ui/chart-controls';
 import { isEmpty } from 'lodash';
 import { LabelProps } from 'src/explore/components/controls/DndColumnSelectControl/types';