Support for logging TLS session keys using SSLKEYLOGFILE environment variable

[Schm1tz1]

This is a follow-up based on the modifications done in PR #2546 . Added handling of SSLKEYLOGFILE environment variable to write the TLS session keys in the same way e.g. cURL does. This is quire an important feature when it comes to traffic analysis with tshark/wireshark. This version was rebased, merged and fixed (style and 1 small bug), tests with the example tools successfully dump the session keys for further analysis.

[cla-assistant[bot]]

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