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Adding lock mechanism to GCS remote backend by suing pubsub Topics.

Concept is imilar to how we create/delete lock file to acquirs/release lock.

We are using remote state-file in our team.We need this in our team to be able to work in a safer way. Our team is small but still this simple change can improve out ability to work together.

Code changes is small.

```hcl
terraform {
  backend "gcs" {
    bucket  = "tf-state-prod"
    path    = "path/terraform.tfstate"
    project = "myproject"
    lockTopic = "remote_lock"
  }
}
```

Only addition will be __lock_topic__. As far as I understand the change is backward compatible and won't break any existing setup. If this change get accepted I will also work/help on moving GCP out of legacy so this change won't cause any extra burden on your team.

I ran the tests to make sure they are all green. All pass locally against my dev project in google cloud.

Also I ran a separate code against pubsub to check if __create__ can be used as lock mechanism. As far as I can tell it is a serial action and usable for lock.

Adding this change will help me greatly and I am hoping that this method might be useful for others too.