[DataGridPro] Use readonly array result for getTreeDataPath.

We do not try to modify the array after receiving it, so let's accept an immutable array.
This is still compatible with existing usage of getTreeDataPath.

packages/x-data-grid-pro/src/models/dataGridProProps.ts

@@ -167,7 +167,7 @@ export interface DataGridProPropsWithoutDefaultValue<R extends GridValidRowModel
    * @param {R} row The row from which we want the path.
    * @returns {string[]} The path to the row.
    */
-  getTreeDataPath?: (row: R) => string[];
+  getTreeDataPath?: (row: R) => ReadonlyArray<string>;
   /**
    * Callback fired when scrolling to the bottom of the grid viewport.
    * @param {GridRowScrollEndParams} params With all properties from [[GridRowScrollEndParams]].

packages/x-data-grid-pro/src/tests/treeData.DataGridPro.test.tsx

@@ -214,7 +214,7 @@ describe('<DataGridPro /> - Tree data', () => {
       ]);
       setProps({
         getTreeDataPath: (row) => [...row.name.split('.').reverse()],
-      } as DataGridProProps);
+      } as Pick<DataGridProProps, 'getTreeDataPath'>);
       expect(getColumnValues(1)).to.deep.equal([
         'A',
         'A.A',