Alerting: Support query_offset in the Prometheus conversion (#102499)

Adds support for rule group-level query_offset in Prometheus to Grafana rule conversion. It allows specifying a time offset for rule evaluation, which gets applied and saved during the conversion.

pkg/services/ngalert/api/tooling/spec.json

@@ -4433,6 +4433,9 @@
         "isPaused": {
           "type": "boolean"
         },
+        "keepFiringFor": {
+          "$ref": "#/definitions/Duration"
+        },
         "labels": {
           "type": "object",
           "additionalProperties": {
@@ -4703,6 +4706,10 @@
         "health": {
           "type": "string"
         },
+        "keepFiringFor": {
+          "type": "number",
+          "format": "double"
+        },
         "labels": {
           "$ref": "#/definitions/Labels"
         },
@@ -7238,9 +7245,22 @@
         "Interval": {
           "$ref": "#/definitions/Duration"
         },
+        "Labels": {
+          "type": "object",
+          "additionalProperties": {
+            "type": "string"
+          }
+        },
+        "Limit": {
+          "type": "integer",
+          "format": "int64"
+        },
         "Name": {
           "type": "string"
         },
+        "QueryOffset": {
+          "type": "string"
+        },
         "Rules": {
           "type": "array",
           "items": {
@@ -7354,6 +7374,10 @@
           "type": "boolean",
           "example": false
         },
+        "keep_firing_for": {
+          "type": "string",
+          "format": "duration"
+        },
         "labels": {
           "type": "object",
           "additionalProperties": {