feat: Implement ETag caching

[vazarkevych]

This PR introduces ETag-based caching (Conditional GET) to the GrowthBook initialization process.

🚀 Key Changes

1. ETag Caching Logic (Conditional GET)

• Separate ETag Storage: Introduced LruETagCache to store ETags independently from the main feature cache.
• Conditional Requests: The initialize method now sends the If-None-Match header if a cached ETag exists.
• 304 Not Modified Handling: Added logic to handle 304 responses from the API, allowing the SDK to reuse stale cache data and refresh its logical TTL without re-downloading the entire payload.

2. Enhanced Caching Wrapper

• Updated saveToCache to wrap data in a payload containing both expires_at (logical TTL) and the actual data.
• Extended the physical cache duration (TTL * 10) to ensure stale data is available for ETag validation even after it technically "expires".