update(ui-grid-header-cell): Enable sorting on grid header cell for enter & space-bar keys.

Issue Description: User is unable to perform sorting actions using keyboard keys on Grid Column Header cell. We have to enable Column Header Menu to perform sorting through Menu option.

Proposed Fix: Updated handleKeyDown method to perform sorting on Enter and Spacebar keys in ui-grid-header-cell.js file.

Testing Details:
Unit tests - PASS
Manual testing - PASS
E2E testing -  PASS

Author: MsDev88

packages/core/src/js/directives/ui-grid-header-cell.js

@@ -161,8 +161,9 @@
             };
 
             $scope.handleKeyDown = function(event) {
-              if (event.keyCode === 32) {
+              if (event.keyCode === 32 || event.keyCode === 13) {
                 event.preventDefault();
+                $scope.handleClick(event);
               }
             };