Add OAuth token, introspect, and revoke endpoints

[Sumeet213]

#260

Summary

Add support for Notion's OAuth 2.0 endpoints to enable public integrations
and token management.

Changes

• Added OAuthEndpoint class with three methods:
  • token() - Exchange authorization codes or refresh tokens
  • introspect() - Validate OAuth tokens
  • revoke() - Revoke OAuth tokens
• Extended authentication to support Basic auth for OAuth client credentials
• Added VCR tests for introspect/revoke and mock tests for token exchange

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7762013) to head (6892cab).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #301      +/-   ##
===========================================
+ Coverage   99.51%   100.00%   +0.48%     
===========================================
  Files           7         7              
  Lines         411       424      +13     
===========================================
+ Hits          409       424      +15     
+ Misses          2         0       -2     

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[ramnes]

LGTM overall, just a few minor points to fix. Thanks!

[ramnes]

Thanks @Sumeet213!