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 99.53%. Comparing base (7762013) to head (7c4b89e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #301      +/-   ##
==========================================
+ Coverage   99.51%   99.53%   +0.01%     
==========================================
  Files           7        7              
  Lines         411      426      +15     
==========================================
+ Hits          409      424      +15     
  Misses          2        2              

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