feat(auth): enhance OAuth2 support with UserOAuthProvider and related functionality

[K-dash]

Summary

• Introduced UserOAuthProvider to facilitate the reuse of refresh tokens generated by the CLI's user-oauth flow in Python.
• Implemented methods for loading OAuth2 refresh tokens from a shared credentials file, improving user experience for end-user scenarios.
• Added tests to validate the functionality of UserOAuthProvider, ensuring correct behavior when loading tokens and handling errors.
• Updated documentation to reflect the new OAuth2 capabilities, including usage examples for both CLI and Python SDK.
• Enhanced the NblmClient to support creating instances with UserOAuthProvider, streamlining the integration of user authentication in applications.

Testing

• Pre-commit hooks passed (automatically checked on commit)

Additional Context

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/nblm-core/src/auth/oauth/mod.rs	94.33%	6 Missing ⚠️

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
+ Coverage   87.56%   88.30%   +0.74%     
==========================================
  Files          40       40              
  Lines        5043     5149     +106     
==========================================
+ Hits         4416     4547     +131     
+ Misses        627      602      -25     

Flag	Coverage Δ
rust	88.30% <94.44%> (+0.74%)	⬆️

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

Files with missing lines	Coverage Δ
crates/nblm-cli/src/util/oauth_bootstrap.rs	62.43% <100.00%> (+0.41%)	⬆️
crates/nblm-core/src/auth/oauth/mod.rs	92.45% <94.33%> (+0.74%)	⬆️

... and 1 file with indirect coverage changes

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