Fake feature: add a "silly_label" variable

[morgante]

This is a fake feature for learning how to contribute to CFT. We should not actually add it.

Desired addition: expose a new (string) variable silly_label which will automatically be added to all bucket labels. For example:

module "gcs_buckets" {
  source  = "terraform-google-modules/cloud-storage/google"
  version = "~> 1.3"
  project_id  = "<PROJECT ID>"
  names = ["first", "second"]
  ...
  silly_label = "awesome"
}

This would label buckets first and second with silly: awesome.

When trying this out, please make sure to add it to examples + tests.