fix: Support discriminator for deserializing polymorphic types

[yoavk]

Partially closes #219.

This fixes deserializing polymorphic types that have the same members, which otherwise would rely on order of types in the union.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 16.66667% with 10 lines in your changes missing coverage. Please review.

Project coverage is 99.49%. Comparing base (aa324fc) to head (ee75c00). Report is 244 commits behind head on main.

Files with missing lines	Patch %	Lines
...penapi_python_client/parser/properties/__init__.py	16.66%	10 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #717      +/-   ##
===========================================
- Coverage   100.00%   99.49%   -0.51%     
===========================================
  Files           49       49              
  Lines         1970     1982      +12     
===========================================
+ Hits          1970     1972       +2     
- Misses           0       10      +10     

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