zmq_proxy_steerable needs properly licensed re-implementation

[brettviren]

Issue description

The next release after 4.3.4 is apparently blocked by the lack of a properly licensed re-implementation of zmq_proxy_steerable.

See this mailing list message for context and guidance on how this can be solved.

https://lists.zeromq.org/pipermail/zeromq-dev/2023-July/033851.html

Pay particular attention to the comment from @bluca

I cannot stress this enough, it must be a clean-room reimplementation
so no prior knowledge of the previous implementation details nor
looking at the previos implementation is allowed (hence why I cannot do
it myself).

See #2376 for the over-arching issue on the re-licensing effort.