feat: Provision to assign multiple payroll cost centers against a single employee

[nabinhait]

Existing:
Earlier we had a provision to assign a payroll cost centre against an Employee via Department/Employee master.

New Feature:
Now you can assign multiple cost centres against an employee. This is a very common use-case, where one employee contributes to multiple departments or cost centers, hence the salary expenses associated with that employee should be distributed against multiple cost centres.

• A new child table "Payroll Cost Centers" has been added inside "Salary Structure Assignment"
  • Here, you can add multiple cost centres with percentages
  • If the table is left blank, it will fetch payroll cost centre from Employee/Department
  • Total percentages should be 100

Docs: https://docs.erpnext.com/compare?wiki_page=docs/v13/user/manual/en/human-resources/payroll-entry&&compare=1a037a4c97 (the docs changes are already live by mistake)

[ruchamahabal]

Not fixing sider issues for frappe.qb. Not helpful

[ruchamahabal]

Raised a PR for fixing unrelated failing tests: #29051

[codecov]

Codecov Report

Merging #29024 (251ed3a) into develop (88df5ce) will decrease coverage by 0.09%.
The diff coverage is 92.98%.

@@             Coverage Diff             @@
##           develop   #29024      +/-   ##
===========================================
- Coverage    55.67%   55.57%   -0.10%     
===========================================
  Files         1128     1129       +1     
  Lines        67335    67379      +44     
===========================================
- Hits         37488    37447      -41     
- Misses       29847    29932      +85     

Impacted Files	Coverage Δ
...yroll/doctype/salary_structure/salary_structure.py	86.56% <ø> (-0.30%)	⬇️
erpnext/hr/doctype/employee/employee.py	67.00% <66.66%> (-0.24%)	⬇️
...tructure_assignment/salary_structure_assignment.py	81.35% <94.73%> (+5.74%)	⬆️
...ctype/employee_cost_center/employee_cost_center.py	100.00% <100.00%> (ø)
...ext/payroll/doctype/payroll_entry/payroll_entry.py	72.44% <100.00%> (+1.65%)	⬆️
...work_order_stock_report/work_order_stock_report.py	50.00% <0.00%> (-50.00%)	⬇️
erpnext/utilities/product.py	14.70% <0.00%> (-35.30%)	⬇️
...wise_balance_history/batch_wise_balance_history.py	68.96% <0.00%> (-24.14%)	⬇️
erpnext/shopping_cart/product_info.py	30.30% <0.00%> (-21.22%)	⬇️
...eport/item_variant_details/item_variant_details.py	84.33% <0.00%> (-3.62%)	⬇️
... and 19 more