Implement AUTH command

[buraksezer]

This PR provides an implementation Redis-style AUTH command.

Supported by cluster client:

func WithCredentials(username, password string) ClusterClientOption {
	return func(cfg *clusterClientConfig) {
		cfg.authentication = &config.Authentication{
			Enabled:  true,
			Username: username,
			Password: password,
		}
	}
}

Embedded client is authenticated by default.

The configuration is quite simple:

authentication:
  enabled: false
  username: "your-username"
  password: "your-password"

Olric returns the following error if the client is unauthorized:

(error) NOAUTH authentication required