Add variable `route_tags` to route definition with instance tags to which this route applies

[damianjaniszewski]

Feature request:
Add route_tags (optional) variable in the module so defined routes will have instance tag specified.
Variable should be passed as tags to resource "google_compute_route" "route" in main.tf.

Example variable def:

variable "route_tags" {
  type    = list(string)
  description = "A list of instance tags to which this route applies."
  default = []
}

[morgante]

This sounds like a reasonable addition and we will add it to the backlog. If you'd like it added sooner, we're also happy to review a pull request.