Support for logging TLS session keys using SSLKEYLOGFILE environment variable

[piotrsmolinski]

SSLKEYLOGFILE environment variable is standard way of declaring the file to write the TLS session keys. This mechanism is used by major web browsers as well as by cURL and other projects. The session key logging is a powerful mechanism to debug network services in such tools as Wireshark.

In Java the same solution is achieved using jSslKeyLog agent (https://svn.code.sf.net/p/jsslkeylog/code). The patch provides the feature to be available in librdkafka based Kafka applications.

The code in the PR was written similar way as the same feature in cURL.

[edenhill]

Please follow the coding style guide: https://github.com/edenhill/librdkafka/blob/master/CONTRIBUTING.md#librdkafka-c-style-guide

[Schm1tz1]

Following up on this. I'd like to assist @piotrsmolinski with this as we both can make us of this feature in several projects and it is probably a really nice feature to have. @edenhill - if you have time for a short call/chat on what's missing and what we need to do to get this PR resolved, please let me know. Also sent you a message on slack!

[cla-assistant[bot]]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

:white_check_mark: Schm1tz1
:x: piotrsmolinski
_{You have signed the CLA already but the status is still pending? Let us recheck it.}