Multiple username, password and client_id

[iegomez]

Hi. Thanks for this great tool!

The thing I'm missing is the possibility to have multiple connection credentials, in order to simulate different users connecting to the broker. My use case is that I have an auth plugin for mosquitto and I'd like to run some benchmarks, maybe see how it stands against jpmens' auth plugin and also what's the real overhead over running regular mosquitto with no plugin, and so I'd love having the option to set a lot of users so that all backends get properly used.

If you are ok with the idea but don't have time to make it happen, I wouldn't mind forking, extending and then making a PR. If you are not interested, just let me know.

Thanks again!