Add Free-Threaded Python Support in 3.14

[mkmoisen]

With the release of Python 3.14, free threaded support (i.e., GIL-less python) is now a sensible option.

Third party packages that use C-Extensions like confluent_kafka however need to have code change to support free-threaded mode.

Here is the documentation on how to change C-Extensions to support free threaded mode:

https://docs.python.org/3.14/howto/free-threading-extensions.html#freethreading-extensions-howto

My understanding is that if a third party application does not support free-threaded python, then applications which import these applications will revert to using the GIL. This means that users are stuck with the GIL until all third party applications have ported to free-threaded compatibility.

May we please put this on the road map for confluent_kafka?

Thank you.

[MSeal]

Thanks for raising the issue @mkmoisen -- I've been a preliminary pass of the C code change requirements. It seems like the code changes will be minimal but testing this will require a bit of preparation work to ensure no deadlocks of segfaults are introduced against the librdkafka layer. I've added it to the roadmap, and will discuss with the team next week.