Add a PkeyClass to API simplifying the openssl and ossl API difference

[Jakuje]

Description

Spin-off from #466, which was kept simple(r)

Checklist

• Test suite updated
• Rustdoc string were added or updated
• CHANGELOG and/or other documentation added or updated
• This is not a code change

Reviewer's checklist:

• Any issues marked for closing are fully addressed
• There is a test suite reasonably covering new functionality or modifications
• This feature/change has adequate documentation added
• A changelog entry is added if the change is significant
• Code conform to coding style that today cannot yet be enforced via the check style test
• Commits have short titles and sensible text
• Doc string are properly updated