Feature Request: Pure UDP Mode (No TCP)

Hi iperf3 team,

First of all, thanks for the amazing iperf3.

A quick request: could we get an option for UDP tests to disable the TCP control channel (e.g. `--pure-udp`)?

This is needed for environments where *any* TCP is blocked (firewalls, TCP-restricted connections etc.), _unlike_ iperf2.

This would be incredibly helpful, since there is no other solution but to use iperf2.

Thanks a lot for considering.