Support `SSLKEYLOGFILE` environment variable

[satyarohith]

To debug encrypted traffic on the client side, utilities like Wireshark need encryption secrets. User applications generate a log file (contains encryption secrets) that can be consumed by Wireshark if you set SSLKEYLOGFILE environment variable or provide an appropriate flag.

The format used in log file is NSS Key Log Format (primary source returns 404).

SSL Key Log generation is supported by the following environments:

• Both Chrome and Firefox support SSLKEYLOGFILE environment variable
• Go provides KeyLogWriter field in config of crypto/tls package https://pkg.go.dev/crypto/tls#example-Config-KeyLogWriter
• Node.js provides --tls-keylog flag and emits keylog event
• cURL supports the environment variable

Edit: the implementation should be straightforward. Rustls already supports the environment variable.