feat: Add certified height to subnet state

[Dfinity-Bjoern]

Let's discuss in the meeting today whether this is something we find acceptable.

[github-actions[bot]]

🤖 Here's your preview: https://fpqpr-eaaaa-aaaak-qclsq-cai.icp0.io/docs

[mraszyk]

Superseded by https://github.com/dfinity/portal/pull/3759