Update Config.Client to redis.UniversalClient for Cluster Support (#24)

[namigaliyev]

This PR updates the Config.Client field in httprateredis.Config from *redis.Client to redis.UniversalClient to enable support for Redis cluster mode via *redis.ClusterClient, addressing issue #24.

Motivation:

• The current *redis.Client type limits httprateredis to single Redis instances, causing type mismatch errors when using *redis.ClusterClient
• redis.UniversalClient is a flexible interface supporting both *redis.Client (single instance), *redis.ClusterClient (cluster mode), and *redis.Ring (sharded Redis), making it suitable for broader use cases.

Changes:

• Changed Config.Client in config.go from *redis.Client to redis.UniversalClient.
• No modifications to rate-limiting logic, as all Redis commands (e.g., INCR, EXPIRE, GET) are supported by redis.UniversalClient.

Impact:

• Enables httprateredis to work seamlessly with Redis cluster mode, addressing community demand in Allow other go-redis client implementations #24.
• Maintains full compatibility with existing *redis.Client usage for single-instance Redis.
• Enhances flexibility for other redis.UniversalClient implementations.

[namigaliyev]

@VojtechVitek @pkieltyka hello, please help to review this, thanks