Clustered Connections

### Describe the feature request

Support for clustered connections like the [NATS](https://github.com/nats-io/nats.net) Broker official clients have. Example:

```
string[] servers = new string[] {
"nats://serverA:1222",
"nats://serverB:1222"
};

Options opts = ConnectionFactory.GetDefaultOptions();
opts.MaxReconnect = 2;
opts.ReconnectWait = 1000;
opts.NoRandomize = true;
opts.Servers = servers;

IConnection c = new ConnectionFactory().CreateConnection(opts);
```

### Which project is your feature request related to?

- ManagedClient

### Additional context
I believe adding clustered support to MQTTnet on top of its current capabilities would place MQTTnet another step ahead of the "competition". To my knowledge there is no MQTT client out there with built-in clustered support.

An MQTT client that saves the developer infrastructure costs and other "Load Balancer" related head aches would be nearly impossible to say no to. In fact, it would make me very excited.