In-memory client cert storage used during cert signing

[todvora]

We don't need any of those certificates stored on FS, they are just used during the signing process. For that, we can use an in-memory implementation.

/nocl

Motivation and Context

Fixes https://github.com/Graylog2/graylog2-server/issues/18587

How Has This Been Tested?

Manually, as an integration test would need a fully secured datanode instead, which we so far can't provide in matrix tests.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Refactoring (non-breaking change)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.