support `SSHKEYLOGFILE`

When serving TLS connections, some applications support setting a `SSHKEYLOGFILE` environment variable so that the application can dump the key material in there, and tools like Wireshark can [pick it up](https://wiki.wireshark.org/TLS) and show the decrypted traffic.

It is possible that this is already possible with Pekko HTTP as well (possibly using a tool such as https://github.com/jsslkeylog/jsslkeylog), or perhaps some changes are needed.

It would be great if someone could look into this and either document how to do it or make an overview of the changes that would be needed.

Start by checking Pekko HTTP's TLS serving configuration and whether SSHKEYLOGFILE is already supported. Compare the possible jsslkeylog approach with Wireshark's TLS key-log requirements. Done means documenting a working setup or producing a scoped overview of the changes needed.

Written by the indexing model from the issue text.