Make (de)serializer configurable via class attribute

[martinn]

A test using AI agents to do this. Need to check it more thoroughly.

Resolves #23

[codecov]

Codecov Report

Attention: Patch coverage is 94.87179% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.6%. Comparing base (c32cdd5) to head (2479c0d).

Files with missing lines	Patch %	Lines
src/quickapi/api.py	85.7%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main     #25     +/-   ##
=======================================
- Coverage   92.6%   90.6%   -2.1%     
=======================================
  Files         16      16             
  Lines        421     395     -26     
  Branches      25      22      -3     
=======================================
- Hits         390     358     -32     
- Misses        26      31      +5     
- Partials       5       6      +1     

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