Add factory pattern to redis_client for multiple client instances

[Copilot]

The redis_client package only supported a singleton pattern via SetConfig()/GetRDB(), preventing applications from creating multiple independent Redis clients (e.g., separate clients for cache, sessions, queues).

Changes

• Added NewRDB(Config) - Factory function to create independent Redis client instances
• Added NewCache(CacheConfig) - Factory function to create independent cache instances with custom configurations (local cache size, TTL, refresh channel)
• Deprecated singleton pattern - SetConfig()/GetRDB()/GetCache() remain for backward compatibility
• Fixed cache invalidation bug - Subscriber now uses cache key from message payload instead of comparing with channel name

Example

// Create multiple independent clients
primaryRDB := redis_client.NewRDB(redis_client.Config{
    Urls:     []string{"primary:6379"},
    Password: "secret1",
})

sessionRDB := redis_client.NewRDB(redis_client.Config{
    Urls:     []string{"session:6379"},
    Password: "secret2",
})

// Create independent cache instances with custom configs
primaryCache := redis_client.NewCache(redis_client.CacheConfig{
    Redis:               primaryRDB,
    RefreshEventChannel: "primary:refresh",
    LocalCacheSize:      2000,
    LocalCacheTTL:       2 * time.Minute,
})

sessionCache := redis_client.NewCache(redis_client.CacheConfig{
    Redis:               sessionRDB,
    RefreshEventChannel: "session:refresh",
})

The factory pattern aligns with other modules in go-webmods (gorm_client, mongo_client, object_storage) and maintains full backward compatibility.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Support creating multiple clients in redis_client</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Support creating multiple clients in redis_client #17

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