Alerting: Expose info about notification delivery errors in a new /receivers endpoint (#55429)

* (WIP) switch to fork AM, first implementation of the API, generate spec

* get receivers avoiding race conditions

* use latest version of our forked AM, tests

* make linter happy, delete TODO comment

* update number of expected paths to += 2

* delete unused endpoint code, code review comments, tests

* Update pkg/services/ngalert/notifier/alertmanager.go

Co-authored-by: Matthew Jacobson <matthew.jacobson@grafana.com>

* remove call to fmt.Println

* clear naming for fields

* shorter variable names in GetReceivers

Co-authored-by: Matthew Jacobson <matthew.jacobson@grafana.com>

pkg/services/ngalert/api/tooling/definitions/alertmanager.go

@@ -142,6 +142,13 @@ import (
 //       400: ValidationError
 //       404: NotFound
 
+// swagger:route GET /api/alertmanager/grafana/config/api/v1/receivers alertmanager RouteGetGrafanaReceivers
+//
+// Get a list of all receivers.
+//
+//     Responses:
+//       200: receivers
+
 // swagger:route POST /api/alertmanager/grafana/config/api/v1/receivers/test alertmanager RoutePostTestGrafanaReceivers
 //
 // Test Grafana managed receivers without saving them.
@@ -403,6 +410,12 @@ type AlertGroup = amv2.AlertGroup
 // swagger:model receiver
 type Receiver = amv2.Receiver
 
+// swagger:model receivers
+type Receivers = []amv2.Receiver
+
+// swagger:model integration
+type Integration = amv2.Integration
+
 // swagger:parameters RouteGetAMAlerts RouteGetAMAlertGroups RouteGetGrafanaAMAlerts RouteGetGrafanaAMAlertGroups
 type AlertsParams struct {