This PR brings support for Redis Cluster in the Messenger component:

• The first commit Support RedisCluster instance allows to pass a RedisCluster object when instanciating the Connection class, which brings support for Redis Cluster without any friction.
• The second commit Support multiple hosts DSN for Redis Cluster is more opiniated and brings a DSN format to configure a Redis Cluster from config/packages/messenger.yaml.

Instanciating Connection with a RedisCluster object:

$redis = new \RedisCluster(null, ['host-01:6379', 'host-02:6379', 'host-03:6379', 'host-04:6379']);
$connection = new Connection([], [], [], $redis);

Configuring a Redis Cluster from YAML:

// config/packages/messenger.yaml

framework:
    messenger:
            metadata:
                default: 'redis://host-01:6379,redis://host-02:6379,redis://host-03:6379'
                lazy: 'redis://host-01:6379?lazy=1,redis://host-02:6379,redis://host-03:6379'

                # Configuration will be `lazy = true` and `auto_setup = true`
                multipleConfig: 'redis://host-01:6379?lazy=1&auto_setup=false,redis://host-02:6379,redis://host-03:6379?auto_setup=true'

This format allows to define multiple hosts for a Redis Cluster and still contains valid URLs. Custom configuration is still supported, it can be specified on only one of the URLs in the DSN (see lazy above). If the user provides multiple configurations on different URLs, they are simply merged with the following code and if an option is defined multiple times then the latest takes precedence (see multipleConfig above).

I understand the way the DSN is handled could not suit you. Please, if you close this PR only for the DSN part, just tell me and I will make a new PR with only the first commit.