Ossl: Ensure libctx is freed on drop

[simo5]

Description

As pointed out in #311 we do not free the Context on drop.
When this code was embedded in kryoptic this was not really necessary as Kryoptic uses a single global context in a lazyly initialized static variable.

But now Ossl exposes the ability to create as many as you want, so they need to be freed on drop.

Checklist

• Test suite updated with functionality tests
• Test suite updated with negative tests
• 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

[Jakuje]

looks straightforward enough

[Jakuje]

still ack. The failed test is the flaky one