dnsdist: Add OCSP stapling (from files) for DoT and DoH

[rgacogne]

Short description

The OCSP responses are loaded from files on startup, and reloaded when the reloadAllCertificates() command is issued.

Fixes #7812.

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)

[chbruyand]

LGTM although I'm not an openssl API expert. I'm just wondering how hard it would be to add a basic unit test ?

[rgacogne]

Thanks for the review!

I'm just wondering how hard it would be to add a basic unit test ?

The issue I encountered was that I could not find any simple way to generate an OCSP response for a certificate using the OpenSSL tools. I did see a (complicated) way to set up an OCSP server responding to queries so I guess we could do that and use the corresponding client to get a response, but that sounded horribly complicated and brittle.
I did all my testing using OCSP responses from Let's Encrypt for valid certificates.

[rgacogne]

Now with regression tests for DoH and DoT OCSP Stapling!
I have to write a bit of code involving undocumented OpenSSL APIs but I hope it will be worth it :-)