feat: Make discount codes & access codes support event_identifiers

[abhinavk96]

Fixes #6250

Short description of what this resolves:

Adds support for fetching discount codes using event identifier, while retaining the functionality of fetching using event id.

[codecov]

Codecov Report

Merging #6251 into development will decrease coverage by 0.02%.
The diff coverage is 14.28%.

@@               Coverage Diff               @@
##           development    #6251      +/-   ##
===============================================
- Coverage        65.86%   65.84%   -0.03%     
===============================================
  Files              286      286              
  Lines            14494    14501       +7     
===============================================
+ Hits              9547     9548       +1     
- Misses            4947     4953       +6

Impacted Files	Coverage Δ
app/api/__init__.py	100% <ø> (ø)	⬆️
app/api/discount_codes.py	22.76% <0%> (-0.31%)	⬇️
app/api/access_codes.py	51.8% <25%> (-1.36%)	⬇️

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