fix: Don't remove the cached tokens when they expire and instead attempt a refresh

[ahmednfwela]

fixes #47

Description

• fix: Attempt to refresh expired tokens instead of throwing them away.
• feat: Use clock package instead of directly calling DateTime.now()
• docs: Fix formatting of the Initialization section

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[codecov]

Codecov Report

Attention: Patch coverage is 84.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 31.23%. Comparing base (9649ee0) to head (b8d442e).

Files	Patch %	Lines
packages/oidc_core/lib/src/utils.dart	0.00%	3 Missing ⚠️
packages/oidc/lib/src/models/event.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #63      +/-   ##
==========================================
- Coverage   33.87%   31.23%   -2.65%     
==========================================
  Files          44       34      -10     
  Lines        1284     1034     -250     
==========================================
- Hits          435      323     -112     
+ Misses        849      711     -138     

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