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 #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)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

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

[moesterheld]

thank you for changing this. lgtm