Alerting: Add rule_query_offset setting for Prometheus rule conversion (#102500)

Adds a new configuration option to specify a time offset for rule evaluation, which gets applied and saved during the Prometheus -> Grafana conversion. For example:

[unified_alerting.prometheus_conversion]
rule_query_offset = 1m

Changing this option affects only the rules imported after the change. If query_offset is set at the group level, it takes precedence over this setting.

Default is set to 1m.

pkg/services/ngalert/api/api_convert_prometheus.go

@@ -463,6 +463,7 @@ func (srv *ConvertPrometheusSrv) convertToGrafanaRuleGroup(
 				IsPaused: pauseAlertRules,
 			},
 			KeepOriginalRuleDefinition: util.Pointer(keepOriginalRuleDefinition),
+			EvaluationOffset:           &srv.cfg.PrometheusConversion.RuleQueryOffset,
 		},
 	)
 	if err != nil {