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]

Codecov Report

Merging #717 (ee75c00) into main (aa324fc) will decrease coverage by 0.50%.
The diff coverage is 16.66%.

@@             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     

Impacted Files	Coverage Δ
...penapi_python_client/parser/properties/__init__.py	97.17% <16.66%> (-2.83%)	⬇️

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[dbanty]

In addition to splitting out the discriminated type from union, test coverage will need to be added in a few ways:

1. An end-to-end test that adds a discriminated type (with partial mapping) to end_to_end_tests/openapi.json
2. Unit tests which call the constructor directly and test for ParseError in the failure conditions (e.g., a discriminator field is not required or not present)

[dbanty]

I try, when at all possible, not to raise anything since it usually results in a poor error message for the consumer—who usually has no idea why the OpenAPI Document is wrong (I should probably add a code style section to the contribution guide).

I think in this case, it might be a good idea to create a new property type DiscriminatedProperty or similar, since we should handle it in a few different ways:

1. We must enforce at parse time (return a ParseError if it fails) that every type within the union is a ModelProperty with a required property matching the discriminator
2. Instead of trying every inner property at runtime, we only attempt the build the one matching the discriminator. So instead of a bunch of try/except, there will be an if statement or, better yet, a dict to look up the correct class.