Add Optional MX Lookup Cache for Enhanced Bulk Email Verification

[emilianocalzada]

Features

• Optional MX Lookup Cache: Disabled by default. Enable it by calling emailverifier.NewVerifier().EnableMXCache(1 * time.Hour).
• Custom TTL: Allows setting a custom Time-To-Live (TTL) for cached MX records.
• Automatic Cleanup: Runs a background ticker to remove expired cache records automatically.

Benefit

• Resource Efficiency: Decreases DNS server load by reusing cached MX records.

Feel free to review and let me know if there are any adjustments needed! close #142