Extend scorecard-backend so that MetricsProviders and Metrics can be optionally disabled by default

Extend the scorecard-backend and scorecard-commons packages so that MetricsProviders and Metrics can be disabled by default.

1. There should be an optional function isEnabled in a MetricsProvider: () => boolean and an optional field enabled: boolean in the Metric type. A missing function or field means it's enabled by default.
2. There should be an optional enabled attribute also in the Config (config.d.ts) for metric providers and metrics to re-enable disable-by-default metrics or to disable specific metric providers or metrics.
3. the backend should ignore disabled metric providers and metrics. Old data should stay in the database. When there are calls that return metrics just based on the database, they should check if the metric is enabled or not.

Start by reading the scorecard-backend and scorecard-commons packages, including config.d.ts, and trace MetricsProvider, Metric, and database-backed metric-returning calls. Confirm how configuration is loaded and where providers and metrics are selected. Done means optional defaults and configuration overrides work, disabled items are ignored, and existing database data remains intact.