perf: Fetching exchange rate on every line item slows down PO (#25345)

* fix: Dont fetch exchange rates for each line item once fetched at parent
`

* perf: Use price list conversion rate from parent

- If price list conversion rate exists in args already from earlier call, use that
- `get_price_list_currency_and_exchange_rate` wont be called for each child row

Co-authored-by: Nabin Hait <nabinhait@gmail.com>

erpnext/public/js/controllers/transaction.js

@@ -1103,6 +1103,8 @@ erpnext.TransactionController = erpnext.taxes_and_totals.extend({
 				to_currency: to_currency,
 				args: args
 			},
+			freeze: true,
+			freeze_message: __("Fetching exchange rates ..."),
 			callback: function(r) {
 				callback(flt(r.message));
 			}