ENH: Add Snøhvit and Mistral to sumo_assets.json

[equinor-ruaj]

Snøhvit and Mistral have been onboarded and configured in Sumo.
This PR adds them to the list of assets recognized by fmu-settings.

Checklist

• Tests added (if not, comment why)
• Test coverage equal or up from main (run pytest with --cov=src/ --cov-report term-missing)
• If not squash merging, every commit passes tests
• Appropriate commit prefix and precise commit message used
• All debug prints and unnecessary comments removed
• Docstrings are correct and updated
• Documentation is updated, if necessary
• Latest main rebased/merged into branch
• Added comments on this PR where appropriate to help reviewers
• Moved issue status on project board
• Checked the boxes in this checklist ✅