Alerting: Refactor encryption and receiver handling for improved clarity (#115959)

* refactor: change GetReceiver to get by UID

to avoid conversions of name to uid back an forth

* refactor: consolidate all encrypt\decrypt functions

* move error from temporary location

pkg/services/ngalert/models/receivers.go

@@ -16,13 +16,6 @@ import (
 	"github.com/grafana/alerting/receivers/schema"
 )
 
-// GetReceiverQuery represents a query for a single receiver.
-type GetReceiverQuery struct {
-	OrgID   int64
-	Name    string
-	Decrypt bool
-}
-
 // GetReceiversQuery represents a query for receiver groups.
 type GetReceiversQuery struct {
 	OrgID   int64