Make self-signed certificates compatible with the Universal Client

Add SAN, SKI, Key Usage fields to the self-signed certificate. The certificate contents roughly mirror what sila_py does.

Does this MR introduce any breaking changes?

No

Checklist:

• The commit messages follow our guidelines
• If necessary, tests have been added
• Docs have been added / updated
• CHANGELOG.md has been updated in the Added/Changed section

Closes #56 (closed)