dnsdist: Add support dumping TLS keys via keyLogFile

[rgacogne]

Short description

This PR:

• moves the DoH ticket keys logic into the DOHAcceptContext object, with the OCSP responses and SSL context objects ;
• adds support dumping TLS keys via the new keyLogFile directive.

The new keyLogFile feature is similar to what various programs do when the SSLKEYLOGFILE environment variable is set, and uses the format described in:

https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/Key_Log_Format

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)