feat: optional `SSLKEYLOGFILE` support

[crepererum]

Motivation

Using SSLKEYLOGFILE is helpful when you want to intercept TLS traffic for debugging and is generally supported by many libraries and browsers. Also see: https://wiki.wireshark.org/TLS#using-the-pre-master-secret

Solution

Add a use_key_log option to server and client TLS configs that -- when set -- will enable rustls's SSLKEYLOGFILE handling.

[crepererum]

Since this is mostly config wiring and relies on the already existing rustls features, does this need a test? If so, where should this test be placed?

[tottoto]

Seems to be related to #893.

[crepererum]

The solution there seems to be: just bypass tonic::transport entirely and write 100+ lines of glue-code to wire up all the components (tower, hyper, rustls) yourself. That seems doable, but somewhat defeats the purpose of tonic, i.e. having an easy-to-use client&server toolkit.