fix(aws-devcontainer): add availability zone selection parameter

This change resolves the TODO on line 157 by adding a new coder_parameter
that allows users to select which AWS availability zone (a, b, c, or d)
their workspace should be deployed in, rather than always defaulting to zone 'a'.

The parameter is set as non-mutable and defaults to 'a' to maintain backward
compatibility with existing workspaces.

registry/coder/templates/aws-devcontainer/main.tf

@@ -142,6 +142,30 @@ data "coder_parameter" "ssh_pubkey" {
   mutable      = false
 }
 
+data "coder_parameter" "availability_zone_suffix" {
+  name         = "availability_zone_suffix"
+  display_name = "Availability Zone"
+  description  = "Which availability zone should your workspace be deployed in?"
+  default      = "a"
+  mutable      = false
+  option {
+    name  = "Zone A"
+    value = "a"
+  }
+  option {
+    name  = "Zone B"
+    value = "b"
+  }
+  option {
+    name  = "Zone C"
+    value = "c"
+  }
+  option {
+    name  = "Zone D"
+    value = "d"
+  }
+}
+
 data "local_sensitive_file" "cache_repo_dockerconfigjson" {
   count    = var.cache_repo_docker_config_path == "" ? 0 : 1
   filename = var.cache_repo_docker_config_path
@@ -154,8 +178,7 @@ data "aws_iam_instance_profile" "vm_instance_profile" {
 
 # Be careful when modifying the below locals!
 locals {
-  # TODO: provide a way to pick the availability zone.
-  aws_availability_zone = "${module.aws_region.value}a"
+  aws_availability_zone = "${module.aws_region.value}${data.coder_parameter.availability_zone_suffix.value}"
 
   hostname   = lower(data.coder_workspace.me.name)
   linux_user = "coder"