Add authentication switches to traffic-gen

[tommairs]

The traffic-gen utility is a very helpful tool for volume testing, but it is lacking the ability to test with smtp_auth.

It would be handy to add the ability to provide a username and password to the command string.
Borrowing from other popular tools, it could look something like this:
/opt/kumomta/sbin/traffic-gen --target <your.sink.server>:25 --auth plain -au <USERNAME> -ap <PASSWORD>

We currently only support AUTH PLAIN, so the --auth switch is not really necessary at this time, but could be made default in the case we add other formats in the future.
The -au (--auth-username) and -ap (--auth-password) could potentially be used also for basic HTTP auth using the --http switch that already exists.

When documenting, you will need to note that --starttls will also be needed when using this.

[cristian-porta]

Hi @tommairs I had implemented this function for my own tests (similar to what I did for the headers later included in #434 ) and only just noticed this issue.
I can commit it and open a PR if you'd like, the code is a bit rough, but it works ( on my PC ;) ) and could serve as a starting point or inspiration for integrating the feature

[wez]

@cristian-porta if you don't mind iterating on it in a PR to whip it into shape, then we'd love to see you open a PR :)