chore: simplify binder APIs by dropping support for generic field extractors

[adriangb]

BREAKING_CHANGE: removed ExtractField and ExtractRepeatedField. Extracting JSON from a form field is no longer supported. Changes to generated OpenAPI.

I think proving out the idea of very generic extractors (e.g. FromRepeatedFormField[FromJson[Model]]) was interesting. It also helped shape some APIs. But I don't think this is a feature I would use all that often, and so I would prefer to get some real world use cases form users before exposing these sorts of APIs. We can add these APIs in the future, but we can't remove them (without breaking changes).

Removing this functionality (and refactoring the binder APIs and implementations) we can arrive at a much simpler system (2 interfaces / ~2 functions to implement the binder API), and reduce the package's codebase by ~15%.

There's a lot more cleanup that needs to be done with tests after this (e.g. coverage for the file extractor); that will be done in subsequent changes.

[codecov-commenter]

Codecov Report

Merging #90 (dcb8871) into main (38b5960) will increase coverage by 0.34%.
The diff coverage is 97.04%.

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   97.66%   98.00%   +0.34%     
==========================================
  Files         154      139      -15     
  Lines        4708     4463     -245     
  Branches      754      697      -57     
==========================================
- Hits         4598     4374     -224     
+ Misses         65       48      -17     
+ Partials       45       41       -4     

Impacted Files	Coverage Δ
...advanced/additional_responses/test_tutorial_002.py	100.00% <ø> (ø)
...advanced/additional_responses/test_tutorial_003.py	100.00% <ø> (ø)
tests/test_docs/advanced/test_root_path.py	100.00% <ø> (ø)
tests/test_docs/tutorial/body/test_tutorial_001.py	100.00% <ø> (ø)
tests/test_docs/tutorial/body/test_tutorial_002.py	100.00% <ø> (ø)
tests/test_docs/tutorial/body/test_tutorial_004.py	100.00% <ø> (ø)
tests/test_docs/tutorial/body/test_tutorial_005.py	100.00% <ø> (ø)
...t_docs/tutorial/cookie_params/test_tutorial_001.py	100.00% <ø> (ø)
...ests/test_docs/tutorial/files/test_tutorial_003.py	100.00% <ø> (ø)
...ests/test_docs/tutorial/files/test_tutorial_004.py	100.00% <ø> (ø)
... and 66 more