Runtime: Check alerts on external tables every 10 minutes by default

[begelundmuller]

• Adds a streaming property to a metrics view's state. If true, it indicates that data may change without reconcile triggering.
• Sets streaming to true for all metrics views that don't reference a source or model.
• Makes alerts that reference a streaming metrics view automatically check every 10 minutes.
• Changes the alert logic to exit early if the watermark has not advanced (to reduce number of expensive alert check queries).
• Also fixes an issue where alerts created through the admin APIs did not have watermark: inherit (so they would use the current time instead of the metric view's watermark).

Closes https://github.com/rilldata/rill-private-issues/issues/212