xmss: mv KeyPair as a container

[tcoratger]

🗒️ Description

As we already have KeyPair as a class in the testing framework, I think that is cleaner to migrate it as a proper XMSS container so that we can reuse it as a return type of the key generation process for example.

🔗 Related Issues or PRs

✅ Checklist

• Ran tox checks to avoid unnecessary CI fails:

  uvx tox

• Considered adding appropriate tests for the changes.
• Considered updating the online docs in the ./docs/ directory.

[unnawut]

makes sense!