take a string like "a b c" and turn that in to "a", "b", "c" in the template

main.tf

variable "input" {
  default = "a b c"
}

resource "template_file" "json" {
  template = "{ \"somekey\": [${list}] }"
  vars {
    list = "${join(", ", formatlist("\\"%s\\"", split(" ", var.input)))}"
  }
}

output "rendered" {
  value = "${template_file.json.rendered}"
}