Alerting: Add backend support for keep_firing_for (#100750)

What is this feature?

This PR introduces a new alert rule configuration option, keep_firing_for (Prometheus documentation).

keep_firing_for prevents alerts from resolving immediately after the alert condition returns to normal. Instead, they transition into a "Recovering" state and are not considered resolved by the Alertmanager. Once the recovery period ends (or after the next evaluation if it is bigger than keep_firing_for), the alert transitions to "Normal" if it doesn't start alerting again:

Before                                          

+----------+     +----------+                    
| Alerting |---->|  Normal  |                    
+----------+     +----------+                    

-----
After

+----------+      +------------+     +----------+
| Alerting |----->| Recovering |---->|  Normal  |
+----------+      +------------+     +----------+                                                 

Why do we need this feature?

This feature prevents flapping alerts by adding a recovery period. This helps avoid false resolutions caused by brief alert

pkg/services/ngalert/api/api_ruler_test.go

@@ -340,6 +340,7 @@ func TestRouteGetRuleByUID(t *testing.T) {
 			gen.WithUniqueGroupIndex(), gen.WithUniqueID(),
 			gen.WithEditorSettingsSimplifiedQueryAndExpressionsSection(true),
 			gen.WithEditorSettingsSimplifiedNotificationsSection(true),
+			gen.WithKeepFiringFor(30*time.Second),
 		).GenerateManyRef(3)
 		require.Len(t, createdRules, 3)
 		ruleStore.PutRule(context.Background(), createdRules...)
@@ -358,6 +359,7 @@ func TestRouteGetRuleByUID(t *testing.T) {
 		require.Equal(t, expectedRule.UID, result.GrafanaManagedAlert.UID)
 		require.Equal(t, expectedRule.RuleGroup, result.GrafanaManagedAlert.RuleGroup)
 		require.Equal(t, expectedRule.Title, result.GrafanaManagedAlert.Title)
+		require.Equal(t, int64(expectedRule.KeepFiringFor), int64(*(result.KeepFiringFor)))
 		require.True(t, result.GrafanaManagedAlert.Metadata.EditorSettings.SimplifiedQueryAndExpressionsSection)
 		require.True(t, result.GrafanaManagedAlert.Metadata.EditorSettings.SimplifiedNotificationsSection)