feat: option to enable/disable multiple Shift Assignments for same dates

[krantheman]

Currently, Shift Assignment allows for multiple Shifts to be assigned to an employee on the same date, provided that the timings don't clash.

This PR introduces an additional Allow Multiple Shift Assignments for Same Date field under HR Settings. As the name suggests, disabling this forbids the user from creating Shift Assignments for an employee with preexisting Assignments on any of the dates within the selected range, and vice versa.

• If Allow Multiple Shift Assignments for Same Date is disabled

• If Allow Multiple Shift Assignments for Same Date is enabled

See: #1086

Docs: https://frappehr.com/docs/v14/en/hr-settings#3-shift-settings

[codecov-commenter]

Codecov Report

Merging #1100 (84a9eab) into develop (79d5504) will increase coverage by 0.02%.
Report is 11 commits behind head on develop.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1100      +/-   ##
===========================================
+ Coverage    73.06%   73.08%   +0.02%     
===========================================
  Files          199      199              
  Lines        10714    10722       +8     
===========================================
+ Hits          7828     7836       +8     
  Misses        2886     2886              

Files	Coverage Δ
...ms/hr/doctype/shift_assignment/shift_assignment.py	94.19% <100.00%> (+0.19%)	⬆️

[ruchamahabal]

Also add docs for this under HR settings + shift assignment