Alerting: rule backtesting API (#57318)

* Implement backtesting engine that can process regular rule specification (with queries to datasource) as well as special kind of rules that have data frame instead of query.
* declare a new API endpoint and model
* add feature toggle `alertingBacktesting`

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

@@ -2384,6 +2384,29 @@
         }
       }
     },
+    "/api/v1/rule/backtest": {
+      "post": {
+        "description": "Test rule",
+        "consumes": [
+          "application/json"
+        ],
+        "produces": [
+          "application/json"
+        ],
+        "tags": [
+          "testing"
+        ],
+        "operationId": "BacktestConfig",
+        "responses": {
+          "200": {
+            "description": "BacktestResult",
+            "schema": {
+              "$ref": "#/definitions/BacktestResult"
+            }
+          }
+        }
+      }
+    },
     "/api/v1/rule/test/grafana": {
       "post": {
         "description": "Test a rule against Grafana ruler",
@@ -2748,6 +2771,60 @@
         }
       }
     },
+    "BacktestConfig": {
+      "type": "object",
+      "properties": {
+        "annotations": {
+          "type": "object",
+          "additionalProperties": {
+            "type": "string"
+          }
+        },
+        "condition": {
+          "type": "string"
+        },
+        "data": {
+          "type": "array",
+          "items": {
+            "$ref": "#/definitions/AlertQuery"
+          }
+        },
+        "for": {
+          "$ref": "#/definitions/Duration"
+        },
+        "from": {
+          "type": "string",
+          "format": "date-time"
+        },
+        "interval": {
+          "$ref": "#/definitions/Duration"
+        },
+        "labels": {
+          "type": "object",
+          "additionalProperties": {
+            "type": "string"
+          }
+        },
+        "no_data_state": {
+          "type": "string",
+          "enum": [
+            "Alerting",
+            "NoData",
+            "OK"
+          ]
+        },
+        "title": {
+          "type": "string"
+        },
+        "to": {
+          "type": "string",
+          "format": "date-time"
+        }
+      }
+    },
+    "BacktestResult": {
+      "$ref": "#/definitions/Frame"
+    },
     "BasicAuth": {
       "type": "object",
       "title": "BasicAuth contains basic HTTP authentication credentials.",