Fix #1051: Replace lru_cache with TTL-based key caching

[ArvinAlizadehGitHub]

Summary

This PR fixes issue #1051 where PyJWKClient with cache_keys=True serves potentially revoked keys indefinitely.

Problem

When cache_keys=True is enabled, PyJWKClient applies @lru_cache to the get_signing_key method. This caches keys permanently until LRU eviction or process restart. If an identity provider removes a key from their JWKS, applications continue accepting tokens signed with that key.

Solution

• Replace lru_cache with TTL-aware caching
• Keys now expire after the configured lifespan (default 300 seconds)
• Maintains full backward compatibility with existing API
• Uses same expiration logic as existing JWKSetCache

Changes

• jwt/jwks_client.py: Replace lru_cache with TTL cache implementation
• tests/test_jwks_client.py: Add test demonstrating the fix works

Testing

All existing tests pass. New test verifies that:

• Old implementation serves cached keys indefinitely
• New implementation properly rejects expired keys

Backward Compatibility

No breaking changes. All existing parameters work identically:

• cache_keys=True still enables individual key caching
• max_cached_keys still limits cache size
• Performance characteristics maintained

Fixes #1051