feat: include template variables in dynamic parameter rendering (#18819)

Closes https://github.com/coder/coder/issues/18671

Template variables now loaded into dynamic parameters.

coderd/testdata/parameters/variables/main.tf

@@ -0,0 +1,30 @@
+// Base case for workspace tags + parameters.
+terraform {
+  required_providers {
+    coder = {
+      source = "coder/coder"
+    }
+    docker = {
+      source  = "kreuzwerker/docker"
+      version = "3.0.2"
+    }
+  }
+}
+
+variable "one" {
+  default = "alice"
+  type    = string
+}
+
+
+data "coder_parameter" "variable_values" {
+  name        = "variable_values"
+  description = "Just to show the variable values"
+  type        = "string"
+  default     = var.one
+
+  option {
+    name  = "one"
+    value = var.one
+  }
+}