refactor: move FIFO valuation logic to separate class

[ankush]

Summary: refactor the way valuation methods are applied (not how they work)

Currently "update_entries_after" is a mighty class that carries the whole stock module's most critical business logic.

Changes:

• valuation.py containing classes for valuation method.
• Abstract away all the nonsensical internal implementation of FIFO queue to a separate class
• Make FIFO queue testable without making stock transactions. New FIFO implementation is pure python object with no DB interactions, so it's independently testable. This means it can also be used outside of update_entries_after in other cases where incoming rate is to be determined :)
• Improve readability of queue implementation. (still sorta WIP)
• Add unit tests for FIFO queue.
• returned popped qty and values for better assertions
• Hypothesis tests! https://hypothesis.works/
• WIP: further Simplifcation and re-evaluation of existing code.

This is how simple the new update stock looks like (ignoring 1 stupid edge case)

fifo_queue = FifoValuation(self.wh_data.stock_queue)
if actual_qty > 0:
    fifo_queue.add_stock(qty=actual_qty, rate=incoming_rate)
else:
    fifo_queue.remove_stock(qty=abs(actual_qty), rate=outgoing_rate)

stock_qty, stock_value = fifo_queue.get_total_stock_and_value()
self.wh_data.stock_queue = fifo_queue.get_state()

[codecov]

Codecov Report

Merging #28947 (2f5be0e) into develop (0ca467a) will increase coverage by 0.09%.
The diff coverage is 97.75%.

@@             Coverage Diff             @@
##           develop   #28947      +/-   ##
===========================================
+ Coverage    55.54%   55.64%   +0.09%     
===========================================
  Files         1127     1128       +1     
  Lines        67241    67281      +40     
===========================================
+ Hits         37351    37437      +86     
+ Misses       29890    29844      -46     

Impacted Files	Coverage Δ
erpnext/stock/stock_ledger.py	85.21% <93.33%> (-0.88%)	⬇️
erpnext/stock/valuation.py	98.64% <98.64%> (ø)
...e/asset_value_adjustment/asset_value_adjustment.py	86.04% <0.00%> (-3.49%)	⬇️
.../hr/doctype/upload_attendance/upload_attendance.py	44.69% <0.00%> (-1.52%)	⬇️
...ice_creation_tool/opening_invoice_creation_tool.py	60.95% <0.00%> (-0.69%)	⬇️
erpnext/stock/get_item_details.py	79.29% <0.00%> (-0.16%)	⬇️
...xt/accounts/doctype/sales_invoice/sales_invoice.py	78.11% <0.00%> (+0.08%)	⬆️
erpnext/payroll/doctype/salary_slip/salary_slip.py	83.99% <0.00%> (+0.26%)	⬆️
erpnext/projects/doctype/project/project.py	54.09% <0.00%> (+0.35%)	⬆️
erpnext/portal/product_configurator/utils.py	33.46% <0.00%> (+0.40%)	⬆️
... and 13 more