Alerting: Add method to reset notification policy tree back to the default (#51934) (#52004)

* Define route and run codegen

* Wire up HTTP layer

* Update API layer and test fakes

* Implement reset of policy tree

* Implement service layer test and authorization bindings

* API layer testing

* Be more specific when injecting settings

(cherry picked from commit fce283d73e)

Co-authored-by: Alexander Weaver <weaver.alex.d@gmail.com>

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

@@ -3780,7 +3780,6 @@
    "type": "object"
   },
   "receiver": {
-   "description": "Receiver receiver",
    "properties": {
     "name": {
      "description": "name",
@@ -4369,6 +4368,24 @@
    }
   },
   "/api/v1/provisioning/policies": {
+   "delete": {
+    "consumes": [
+     "application/json"
+    ],
+    "operationId": "RouteResetPolicyTree",
+    "responses": {
+     "202": {
+      "description": "Ack",
+      "schema": {
+       "$ref": "#/definitions/Ack"
+      }
+     }
+    },
+    "summary": "Clears the notification policy tree.",
+    "tags": [
+     "provisioning"
+    ]
+   },
    "get": {
     "operationId": "RouteGetPolicyTree",
     "responses": {