feat: leeway support + strict time integrity

[ixunio]

• add leeway for 'iat', 'nbf', 'exp' validation against now. Default leeway = 5s. Can be changed with JWTClaims.set_leeway().
• By default, 'iat' consistency will be checked: if 'iat' in the future, raise ValueError. Can be disabled with JWTClaims.disallow_future_iat(), can be reenabled with JWTClaims.allow_future_iat().

[codecov]

Codecov Report

❌ Patch coverage is 98.58657% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.07%. Comparing base (5d8f00c) to head (e31b75c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
superjwt/utils.py	55.55%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
+ Coverage   95.96%   96.07%   +0.10%     
==========================================
  Files          14       14              
  Lines        1536     1629      +93     
  Branches       95       98       +3     
==========================================
+ Hits         1474     1565      +91     
- Misses         41       42       +1     
- Partials       21       22       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.