feat: Unit Price Items (backport #46214)

[mergify]

Depends on #47537

Docs:

• Selling Settings: https://docs.frappe.io/erpnext/user/manual/en/selling-settings?editWiki=1&wikiPagePatch=77tg17utvv
• Buying Settings: https://docs.frappe.io/erpnext/user/manual/en/buying-settings?editWiki=1&wikiPagePatch=0hopgllcfg

Closes #36012 (Refer for more Info)

Concept

• Unit Price Items or Unit Price Contracts are a way to sell/agree upon a list of items whose quantity cannot be determined at the time of the sale but Rate/Unit Price can.
• The quantity is determined once the job is fully or partially done and the agreement is to bill as per the Unit Price

Examples:

• Sales Order for a Consultation firm:

Item	Rate	Qty
Consulting (per hour)	$60	?

• Sales Order for a Construction Company for Road Work:

Item	Rate	Qty
Labour (per hour)	$60	?
Subcontracting/Partner Costs (per hour)	$40	?

• The Sales Order/Quotation in both cases is agreed upon based on the unit cost/rate.
• The ? Quantities will be determined once the job is done E.g. The hours put in for road work will be updated once half or all of the work is done. Invoicing is done on the basis of actual quantities.

Setup and Working

• 
• 
• We now flag the record as has_unit_price_items if allowed via Settings and if it has any 0 qty items
• We don't maintain row wise flags anymore. The user is free to leave the qty as 0. Nothing more
• They will be reminded that the record contains unit price items
  
• A row is considered a unit price item only as long as it has 0 qty
• Mapping to stock documents: Qty is mapped as is (0) until some qty is set in the Order, then it acts like a normal SO/PO row
• Why 0 qty ? 1 Qty might be confusing as users might assume 1 qty is being sold in actuality. Also this makes handling billing, etc. easy with the least amount of code changes

Updating the SO/PO

Users can decide to manage their Transactions in two ways

1. Keep updating the Qty in the Order ["Sales Order", "Purchase Order"]
   • Once a portion of work is done, users can update the qty in the SO/PO -> Then create SI/PI for the updated qty
2. Keep the Qty 0 [SO, PO, RFQ, SQ, Quotation]
   • For those who might think constant updation SO is adding friction OR are not particular about tracking OR cannot update quantities in certain documents after submission, you can leave the quantities to be 0.
   • They can keep making Invoices with actual quantities against the SO/PO with 0 qty. The Billed % will always show as 0
   • The SO/PO will have to be manually Closed. Since there's no way to know when the Order is truly Completed.

TODO:

• Extend to Purchase
• Tests
• Docs

Revival of #36024 in a hopefully cleaner way

---

This is an automatic backport of pull request #46214 done by Mergify.

[marination]

TODO:

• Conflicts
• Include fix: Relabel unit price settings for more clarity #47432
• Include fix(portal): User cannot create 0 qty SQ from RFQ #47438
• Backpprt refactor: Consolidate duplicate zero-quantity Items checks for transactions. #38530

[marination]

Linter failure is unrelated to the changes