CSI: volume and plugin allocations in the API (#8590)

* command/agent/csi_endpoint: explicitly convert to API structs, and convert allocs for single object get endpoints

api/tasks.go

@@ -925,6 +925,7 @@ type TaskEvent struct {
 	Time           int64
 	DisplayMessage string
 	Details        map[string]string
+	Message        string
 	// DEPRECATION NOTICE: The following fields are all deprecated. see TaskEvent struct in structs.go for details.
 	FailsTask        bool
 	RestartReason    string
@@ -933,7 +934,6 @@ type TaskEvent struct {
 	DriverMessage    string
 	ExitCode         int
 	Signal           int
-	Message          string
 	KillReason       string
 	KillTimeout      time.Duration
 	KillError        string