feat: Add PKCE OAuth flow and refactor OAuth structure

[chyroc]

• Implemented PKCE OAuth flow including authorization url generation, code-to-token exchange, and token refresh
• Refactored JWT OAuth by creating a common OAuthApp base class
• Ensured both PKCE OAuth app and JWT OAuth app inherit from the new OAuthApp base class for improved code reuse and maintainability

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 52.11268% with 34 lines in your changes missing coverage. Please review.

Project coverage is 74.59%. Comparing base (f860b5f) to head (308b453). Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
cozepy/auth/__init__.py	54.54%	25 Missing :warning:
tests/config.py	28.57%	5 Missing :warning:
cozepy/util.py	33.33%	4 Missing :warning:

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   76.51%   74.59%   -1.93%     
==========================================
  Files          30       30              
  Lines        1005     1055      +50     
==========================================
+ Hits          769      787      +18     
- Misses        236      268      +32     

Flag	Coverage Δ
	74.59% <52.11%> (-1.93%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
cozepy/__init__.py	100.00% <100.00%> (ø)
tests/test_auth.py	100.00% <100.00%> (ø)
cozepy/util.py	69.23% <33.33%> (-30.77%)	:arrow_down:
tests/config.py	66.66% <28.57%> (-16.67%)	:arrow_down:
cozepy/auth/__init__.py	80.00% <54.54%> (-18.88%)	:arrow_down: