Allow to provide a user password for non interactive sessions

# Enhancement Request

* Current behavior
When using authentication the iperf3 client will ask for the password on the command line. While this is very useful for manual testing it makes automated testing using authentication (e.g. in a cronjob) very hard.

* Desired behavior
Give an option to specify the password. Either as a new command line option, or attempt to read it from a (set of) hardcoded file(s) first and only ask for it once non was found.

I'd be happy to provide an implementation of the second approach.