perf: add in some indexes (backport frappe#40590) (frappe#40604)

* perf: add in some indexes (frappe#40590)

`Sales Invoice Item.purchase_order`
`Delivery Note Item.purchase_order`

Signed-off-by: Akhil Narang <me@akhilnarang.dev>
(cherry picked from commit 1cd38c8)

# Conflicts:
#	erpnext/selling/doctype/sales_order_item/sales_order_item.json
#	erpnext/stock/doctype/delivery_note_item/delivery_note_item.json

* chore: `conflicts`

* chore: `conflicts`

---------

Co-authored-by: Akhil Narang <me@akhilnarang.dev>
Co-authored-by: s-aga-r <sagarsharma.s312@gmail.com>

erpnext/selling/doctype/sales_order_item/sales_order_item.json

@@ -832,7 +832,8 @@
    "label": "Purchase Order",
    "options": "Purchase Order",
    "print_hide": 1,
-   "read_only": 1
+   "read_only": 1,
+   "search_index": 1
   },
   {
    "fieldname": "column_break_89",
@@ -875,7 +876,7 @@
  "idx": 1,
  "istable": 1,
  "links": [],
- "modified": "2023-11-24 19:07:17.715231",
+ "modified": "2024-03-21 18:15:56.625005",
  "modified_by": "Administrator",
  "module": "Selling",
  "name": "Sales Order Item",

erpnext/stock/doctype/delivery_note_item/delivery_note_item.json

@@ -809,7 +809,8 @@
    "label": "Purchase Order",
    "options": "Purchase Order",
    "print_hide": 1,
-   "read_only": 1
+   "read_only": 1,
+   "search_index": 1
   },
   {
    "fieldname": "column_break_82",
@@ -870,7 +871,7 @@
  "index_web_pages_for_search": 1,
  "istable": 1,
  "links": [],
- "modified": "2023-11-14 18:37:38.638144",
+ "modified": "2024-03-21 18:15:07.603672",
  "modified_by": "Administrator",
  "module": "Stock",
  "name": "Delivery Note Item",