feat: add ResponseCache utility class

[LifeJiggy]

---

PR FIXED: Clean ResponseCache Only

Hi @bbatha! 👋

PR #74 was automatically closed when I initially force-pushed with too many files (632 files instead of 3).

I've now fixed it completely:

• Deleted the problematic branch
• Created a fresh, clean branch from main
• Only included ResponseCache code (no CLI, no validators, no other features)
• All tests passing (5/5 tests )

Removed All Unwanted Features:

• CLI tool - Completely removed
• Key validator - Completely removed
• Rate limiter - Completely removed
• All other utilities - Only ResponseCache remains

Current PR Contains:

• ResponseCache class with TTL support and LRU eviction
• Thread-safe caching with configurable max size
• Comprehensive test suite (5 tests, all passing ✅)
• Clean imports in _utils/__init__.py
• Zero external dependencies

PR Stats (Now Perfectly Clean):

• 3 files changed
• 162 lines added (no deletions)
• 100% focused on response caching

ResponseCache Benefits:

• TTL-based expiration for cached responses
• LRU eviction when cache reaches max size
• Thread-safe for concurrent access
• Configurable parameters (max_size, default_ttl)
• Easy integration with Gradient client
• Production-ready with comprehensive testing

📝 Usage Example:

from gradient._utils import ResponseCache

cache = ResponseCache(max_size=100, default_ttl=300)

# Cache a response
cache.set("GET", "https://api.example.com/data", response_data, ttl=600)

# Retrieve from cache
cached_data = cache.get("GET", "https://api.example.com/data")

This PR now contains ONLY the ResponseCache functionality you wanted to review. No extra features, no distractions - just clean, focused caching code.

Ready for your review of the ResponseCache implementation!