Dynamically select from available backends

[semohr]

• Validation backends (jsonschema, pydantic) are now optional extras and must be explicitly installed. Auto-detection logic picks the best available backend (Pydantic preferred). The default jsonschema dependency is removed from core requirements.
• Expanded and clarified documentation on validation backends, including a new docs/validation.md with a feature comparison, installation instructions, and error handling examples.

Performance and Benchmarks:

• Added a new benchmark script (benchmarks/validation.py) to measure validation speed, dependency size, and cold import time for both backends.
• Optimized the asdict_with_aliases implementation: added an LRU cache for alias detection and improved the alias dict factory to O(N) complexity.

CLI and Error Handling:

• CLI now handles single validation errors (ConfigurationError) in addition to multiple errors, improving UX for config validation.

[codecov]

Codecov Report

❌ Patch coverage is 98.61111% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.87%. Comparing base (b7c7f30) to head (c2fd34c).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/eyconf/validation/backends/__init__.py	96.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
- Coverage   95.95%   95.87%   -0.08%     
==========================================
  Files          18       18              
  Lines        1062     1115      +53     
==========================================
+ Hits         1019     1069      +50     
- Misses         43       46       +3     

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