feat: add validation to availability_zone_count (#122)

This ensures that availability_zone_count is only set to allowed values, see https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/elasticsearch_domain#availability_zone_count.

variables.tf

@@ -116,6 +116,11 @@ variable "availability_zone_count" {
   type        = number
   default     = 2
   description = "Number of Availability Zones for the domain to use."
+
+  validation {
+    condition     = contains([2, 3], var.availability_zone_count)
+    error_message = "The availibility zone count must be 2 or 3."
+  }
 }
 
 variable "ebs_volume_size" {
@@ -357,4 +362,4 @@ variable "custom_endpoint_certificate_arn" {
   type        = string
   description = "ACM certificate ARN for custom endpoint."
   default     = ""
-}
+}