feat: add OAuth PKCE

[J0]

What kind of change does this PR introduce?

To mirror the JS lib. Probably first of a few PRs

cc: @silentworks

[codecov]

Codecov Report

Attention: 35 lines in your changes are missing coverage. Please review.

Comparison is base (160b671) 45.68% compared to head (364292c) 45.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #331      +/-   ##
==========================================
- Coverage   45.68%   45.37%   -0.32%     
==========================================
  Files          23       23              
  Lines        1902     1955      +53     
==========================================
+ Hits          869      887      +18     
- Misses       1033     1068      +35     

Files	Coverage Δ
gotrue/types.py	99.05% <100.00%> (+0.02%)	⬆️
gotrue/helpers.py	67.53% <46.15%> (-4.35%)	⬇️
gotrue/_async/gotrue_client.py	29.68% <12.50%> (-0.70%)	⬇️
gotrue/_sync/gotrue_client.py	29.68% <12.50%> (-0.70%)	⬇️

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[silentworks]

LGTM