Alerting: Receivers API (read only endpoints) (#81751)

* Add single receiver method

* Add receiver permissions

* Add single/multi GET endpoints for receivers

* Remove stable tag from time intervals

See end of PR description here: https://github.com/grafana/grafana/pull/81672

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

@@ -0,0 +1,56 @@
+package definitions
+
+// swagger:route GET /v1/notifications/receivers/{Name} notifications RouteGetReceiver
+//
+// Get a receiver by name.
+//
+//    Responses:
+//      200: GetReceiverResponse
+// 	    403: PermissionDenied
+//      404: NotFound
+
+// swagger:route GET /v1/notifications/receivers notifications RouteGetReceivers
+//
+// Get all receivers.
+//
+//    Responses:
+//      200: GetReceiversResponse
+//      403: PermissionDenied
+
+// swagger:parameters RouteGetReceiver
+type GetReceiverParams struct {
+	// in:path
+	// required: true
+	Name string `json:"name"`
+	// in:query
+	// required: false
+	Decrypt bool `json:"decrypt"`
+}
+
+// swagger:parameters RouteGetReceivers
+type GetReceiversParams struct {
+	// in:query
+	// required: false
+	Names []string `json:"names"`
+	// in:query
+	// required: false
+	Limit int `json:"limit"`
+	// in:query
+	// required: false
+	Offset int `json:"offset"`
+	// in:query
+	// required: false
+	Decrypt bool `json:"decrypt"`
+}
+
+// swagger:response GetReceiverResponse
+type GetReceiverResponse struct {
+	// in:body
+	Body GettableApiReceiver
+}
+
+// swagger:response GetReceiversResponse
+type GetReceiversResponse struct {
+	// in:body
+	Body []GettableApiReceiver
+}

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

@@ -1,6 +1,6 @@
 package definitions
 
-// swagger:route GET /v1/notifications/time-intervals notifications stable RouteNotificationsGetTimeIntervals
+// swagger:route GET /v1/notifications/time-intervals notifications RouteNotificationsGetTimeIntervals
 //
 // Get all the time intervals
 //
@@ -8,7 +8,7 @@ package definitions
 //       200: GetAllIntervalsResponse
 //       403: ForbiddenError
 
-// swagger:route GET /v1/notifications/time-intervals/{name} notifications stable RouteNotificationsGetTimeInterval
+// swagger:route GET /v1/notifications/time-intervals/{name} notifications RouteNotificationsGetTimeInterval
 //
 // Get a time interval by name.
 //
@@ -17,7 +17,7 @@ package definitions
 //       404: NotFound
 //       403: ForbiddenError
 
-// swagger:parameters stable RouteNotificationsGetTimeInterval
+// swagger:parameters RouteNotificationsGetTimeInterval
 type RouteTimeIntervalNameParam struct {
 	// Time interval name
 	// in:path