add binding for X509_digest

[schubev]

Adds bindings for the X509_digest function which retrieves a certificate’s fingerprint using one of a collection of digest algorithms.

Questions

• SHA1, SHA256 and SHA384 satisfy my needs but it would be easy for me to add some more algorithms to the PR: do you think it is necessary? On the other hand it would still be easy to add some later on, on an as-needed basis.
• It seems this repository has no tests. I personally use Alcotest and included the tests I used to build the feature in the PR, but I understand choosing a test framework may be out of scope for this PR so I can definitely just keep those in my fork.