feat(subcontracting): Added provision to create multiple Subcontracting Orders against a single Purchase Order (backport #44711)

[mergify]

Made changes to fulfil issue #43986

Current flow: Only a single Subcontracting Order could be made against a Subcontracted Purchase Order

New flow: User can now create multiple Subcontracting Orders against a single Purchase Order

1. Checks for existing Subcontracting Order has been removed
2. Upon creating a new Subcontracting Order, a new check has been added which will throw and error if user is trying to create a new SO against a PO which has been fully subcontracted

How to use: When creating a Subcontracting Order, user now has the ability to change the quantity of finished good item, hence enabling them to create a SO which partly fulfils the corresponding PO

Notes:

1. An error will be thrown if quantity entered is higher than available subcontracting quantity (fetched from the PO)
2. Upon creating a new Subcontracted Order, the line items will by default have the quantity as the remaining/available quantity to subcontract

https://docs.frappe.io/erpnext/user/manual/en/subcontracting

---

This is an automatic backport of pull request #44711 done by Mergify.

[mergify]

Cherry-pick of 3eba6bf has failed:

On branch mergify/bp/version-15-hotfix/pr-44711
Your branch is up to date with 'origin/version-15-hotfix'.

You are currently cherry-picking commit 3eba6bf3dd.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   erpnext/buying/doctype/purchase_order/purchase_order.py
	modified:   erpnext/buying/doctype/purchase_order_item/purchase_order_item.py
	modified:   erpnext/controllers/subcontracting_controller.py
	modified:   erpnext/controllers/tests/test_subcontracting_controller.py
	modified:   erpnext/subcontracting/doctype/subcontracting_order/subcontracting_order.js
	modified:   erpnext/subcontracting/doctype/subcontracting_order/subcontracting_order.py
	modified:   erpnext/subcontracting/doctype/subcontracting_order/test_subcontracting_order.py
	modified:   erpnext/subcontracting/doctype/subcontracting_order_item/subcontracting_order_item.py
	modified:   erpnext/subcontracting/doctype/subcontracting_order_service_item/subcontracting_order_service_item.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/buying/doctype/purchase_order/purchase_order.js
	both modified:   erpnext/buying/doctype/purchase_order/test_purchase_order.py
	both modified:   erpnext/buying/doctype/purchase_order_item/purchase_order_item.json
	both modified:   erpnext/subcontracting/doctype/subcontracting_order_item/subcontracting_order_item.json
	both modified:   erpnext/subcontracting/doctype/subcontracting_order_service_item/subcontracting_order_service_item.json

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally