keylogfile: generalize with a callback

This supersedes !1112 (closed) after the discussion about not making the mechanism QUIC specific.

Instead this refactors the keylogfile mechanism by adding a callback to get notified when a new secret is derived and installed. That way, consumers can implement custom logging feature per session, which is particularly useful in QUIC implementation.

Note that the code is sufficiently covered by the existing keylogfile tests.

Fixes #852 (closed).

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)
• CI timeout is 2h or higher (see Settings/CICD/General pipelines/Timeout)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code