perf: Index sales_order_item in Pick list item (backport #35735) (#35736)

* perf: Index sales_order_item in Pick list item (#35735)

- `get_picked_items_qty` does full table scan
- because it also locks, it does full table lock.

(cherry picked from commit 07d748c290)

# Conflicts:
#	erpnext/stock/doctype/pick_list_item/pick_list_item.json

* chore: conflicts

---------

Co-authored-by: Ankush Menat <ankush@frappe.io>

erpnext/stock/doctype/pick_list_item/pick_list_item.json

@@ -149,7 +149,8 @@
    "fieldtype": "Data",
    "hidden": 1,
    "label": "Sales Order Item",
-   "read_only": 1
+   "read_only": 1,
+   "search_index": 1
   },
   {
    "fieldname": "serial_no_and_batch_section",
@@ -191,7 +192,7 @@
  ],
  "istable": 1,
  "links": [],
- "modified": "2022-04-22 05:27:38.497997",
+ "modified": "2023-06-16 14:05:51.719959",
  "modified_by": "Administrator",
  "module": "Stock",
  "name": "Pick List Item",