PLT-0 - Make redrive policy optional

* make redrive policy optional as you now can not create a subscription without redrive policy

variables.tf

@@ -7,7 +7,7 @@ variable "sns_topic_subscriptions" {
     endpoint_auto_confirms = bool
     raw_message_delivery   = bool
     filter_policy          = string
-    redrive_policy         = string
+    redrive_policy         = optional(string)
   }))
   default     = []
   description = "SNS Subscriptions"