feat: Allow order amount override from maintainers

[iamareebjamal]

Fixes fossasia/open-event-frontend#5184

[codecov]

Codecov Report

Merging #7377 into development will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           development    #7377   +/-   ##
============================================
  Coverage        64.23%   64.24%           
============================================
  Files              259      259           
  Lines            13130    13133    +3     
============================================
+ Hits              8434     8437    +3     
  Misses            4696     4696           

Impacted Files	Coverage Δ
app/api/custom/orders.py	37.71% <100.00%> (+0.72%)	⬆️
app/api/custom/schema/order_amount.py	100.00% <100.00%> (ø)

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