Traits must be public to import them externally

[dmulder]

The functions implemented by these traits in SoftHsm were private and could not be imported.

(Refers|Fixes) #

Checklist

• [x] This pr contains no AI generated code
• [x] cargo fmt has been run
• [x] cargo clippy has been run and there's no issues
• [x] cargo test has been run and passes