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/tests/api/alerting/test-data/rulegroup-3-get.json

@@ -5,6 +5,7 @@
     {
       "expr": "",
       "for": "5m",
+      "keep_firing_for": "0s",
       "labels": {
         "label1": "test-label"
       },
@@ -51,6 +52,7 @@
     {
       "expr": "",
       "for": "5m",
+      "keep_firing_for": "0s",
       "labels": {
         "label1": "test-label"
       },