Make argument types in MqttOptions::new independent

[dbrgn]

The two argument types need to be independent. Otherwise the instance cannot be created with both a String and a &'static str instance.

An example that doesn't currently compile:

let client_id = "hello".to_string();
let client = MqttOptions::new(client_id, "eu.thethings.network:1883");

[tekjar]

Thanks :)