Alerting: new min_interval_seconds options to enforce a minimum eval frequency (#21188)

* add min_interval_seconds setting to alerting config

It will let operator enforce a minimum time for the scheduler to enqueue evaluations

* Introduce UI modifications

* Update docs

Co-authored-by: Martin <uepoch@users.noreply.github.com>

packages/grafana-runtime/src/config.ts

@@ -34,6 +34,7 @@ export class GrafanaBootConfig {
   alertingEnabled = false;
   alertingErrorOrTimeout = '';
   alertingNoDataOrNullValues = '';
+  alertingMinInterval = 1;
   authProxyEnabled = false;
   exploreEnabled = false;
   ldapEnabled = false;