refactor: Made auth_headers and auth_params of APIAuthenticatorBase simple instance attributes instead of decorated properties

[edgarrmondragon]

Updated API docs (press d on the page to toggle the diff view):

• https://meltano-sdk--2596.org.readthedocs.build/en/2596/classes/singer_sdk.authenticators.OAuthAuthenticator.html#singer_sdk.authenticators.OAuthAuthenticator
• https://meltano-sdk--2596.org.readthedocs.build/en/2596/classes/singer_sdk.authenticators.APIAuthenticatorBase.html#singer_sdk.authenticators.APIAuthenticatorBase

Closes #925

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📚 Documentation preview 📚: https://meltano-sdk--2596.org.readthedocs.build/en/2596/

[codspeed-hq]

CodSpeed Performance Report

Merging #2596 will not alter performance

_{Comparing edgarrmondragon/refactor/auth-header-params-attrs (0270bcf) with main (75eb494)}

Summary

✅ 6 untouched benchmarks

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 16 lines in your changes missing coverage. Please review.

Project coverage is 89.48%. Comparing base (75eb494) to head (0270bcf).
Report is 120 commits behind head on main.

Files with missing lines	Patch %	Lines
singer_sdk/authenticators.py	20.00%	14 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2596   +/-   ##
=======================================
  Coverage   89.47%   89.48%           
=======================================
  Files          58       58           
  Lines        4800     4792    -8     
  Branches      937      934    -3     
=======================================
- Hits         4295     4288    -7     
+ Misses        352      351    -1     
  Partials      153      153           

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