Alerting: New API endpoint GET /api/v1/ngalert (#55134)

* declare new endpoint /api/v1/ngalert
* add authorization for new path
* add request handler for the new path

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

@@ -1757,6 +1757,26 @@
         }
       }
     },
+    "/api/v1/ngalert": {
+      "get": {
+        "description": "Get the status of the alerting engine",
+        "produces": [
+          "application/json"
+        ],
+        "tags": [
+          "configuration"
+        ],
+        "operationId": "RouteGetStatus",
+        "responses": {
+          "200": {
+            "description": "AlertingStatus",
+            "schema": {
+              "$ref": "#/definitions/AlertingStatus"
+            }
+          }
+        }
+      }
+    },
     "/api/v1/provisioning/alert-rules": {
       "post": {
         "consumes": [
@@ -2733,6 +2753,19 @@
         }
       }
     },
+    "AlertingStatus": {
+      "type": "object",
+      "properties": {
+        "alertmanagersChoice": {
+          "type": "string",
+          "enum": [
+            "all",
+            "internal",
+            "external"
+          ]
+        }
+      }
+    },
     "ApiRuleNode": {
       "type": "object",
       "properties": {