fix: allow arbitary items in Quotation and Sales Order

[SvbZ3r0]

Arbitary items are required while quoting for an engineered-to-order product. There can't be an item in your database for every request your customer makes.
Allowing arbitary items in Sales Orders follows the same logic. Billing of arbitary items is already allowed.

closes #33429

The Buying side of this issue can possibly be implemented in a similar fashion, but I'm not sure if it'll break anything.

[codecov]

Codecov Report

Merging #33430 (d12cd67) into develop (9858d9d) will increase coverage by 0.02%.
The diff coverage is n/a.

❗ Current head d12cd67 differs from pull request most recent head f6db520. Consider uploading reports for the commit f6db520 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #33430      +/-   ##
===========================================
+ Coverage    64.07%   64.10%   +0.02%     
===========================================
  Files          818      818              
  Lines        58691    58670      -21     
===========================================
+ Hits         37607    37609       +2     
+ Misses       21084    21061      -23     

Impacted Files	Coverage Δ
...customer_ledger_summary/customer_ledger_summary.py	0.00% <0.00%> (ø)
erpnext/selling/doctype/sales_order/sales_order.py	80.28% <0.00%> (+0.07%)	⬆️

[SvbZ3r0]

@deepeshgarg007 can we merge this into v14? I'd forgotten about this since i have a custom app doing this for me