Allow installing as a service on macOS via brew

Hello! I have Mac Minis running as network management hosts on my customers' networks, and would like to install iperf3 as a service on these, so I can test network performance from different points in their networks.

Considering iperf3 is already in Homebrew, I believe this can be achieved by adding a `service` block to [the formula file](https://github.com/Homebrew/homebrew-core/blob/main/Formula/i/iperf3.rb); example from colima [here](https://github.com/Homebrew/homebrew-core/blob/08cab3b62728fbecf48c49fc80811ef1e0cc8df8/Formula/c/colima.rb#L39-L46).

I'm not sure how the formula files are maintained, so I thought I'd pitch it here and ask for direction, if there's anything I can do to help make this happen.

The alternative is writing a plist file, and figuring out how to use launchctl to register it as a service, which doesn't scale as well.