refactor(grids): mark primaryKey as string

Author: damyanpetev

projects/igniteui-angular/src/lib/grids/common/grid.interface.ts

@@ -671,7 +671,7 @@ export interface GridType extends IGridDataBindable {
      /** Indicates whether rows in the grid can be dragged. If te value is true, the rows can be dragged */
     rowDraggable: boolean;
     /** Represents the unique primary key used for identifying rows in the grid */
-    primaryKey: any;
+    primaryKey: string;
     /** Represents the unique identifier of the grid. */
     id: string;
     /** The height of the visible rows in the grid. */

projects/igniteui-angular/src/lib/grids/grid-base.directive.ts

@@ -436,7 +436,7 @@ export abstract class IgxGridBaseDirective implements GridType,
      */
     @WatchChanges()
     @Input()
-    public primaryKey: any;
+    public primaryKey: string;
 
     /**
      * Gets/Sets a unique values strategy used by the Excel Style Filtering