vibe-stream now supports SSLKEYLOGFILE

[Steven Schveighoffer]

Just added a new feature to vibe-stream for anyone doing TLS 
debugging with wireshark.

There is a feature supported on cURL and most browsers known as 
[SSLKEYLOGFILE](https://www.ietf.org/archive/id/draft-thomson-tls-keylogfile-00.html)

A quick tutorial on how to use it in cURL is 
[here](https://everything.curl.dev/usingcurl/tls/sslkeylogfile.html).

For the updated 
[vibe-stream](https://code.dlang.org/packages/vibe-stream) 
(1.3.0), you can enable the feature (that it uses the environment 
variable to set up a key log file) by enabling version 
`VibeKeylogFromEnvironment`.

You can also set up key logging by assigning your own function to 
the new field (on OpenSSLContext) called `keylogCallback`. This 
may not be in docs yet, I'm not sure how they update on vibed.org.

Note that this feature is only available with libssl version 
1.1.1 or higher. It does not work on other libraries or prior 
versions (though 1.1.1 is quite old).

Enjoy!

-Steve