Store the underlying model's last refresh time in the metrics view state

[begelundmuller]

Changes:

• Adds a model_refreshed_on field on the metrics view state.
• During validation, it copies the metrics view's underlying model's refreshed_on into model_refreshed_on.
• model_refreshed_on will be empty for metrics views that are backed by external tables
• The model_refreshed_on field will not be updated if the metrics view fails validation and keeps serving the old valid_spec.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated
• Intend to cherry-pick into the release branch
• I'm proud of this work!