Add GCS source support for modules

[davewongillies]

Adds support for GCS style source URL for modules, as implemented in hashicorp/go-getter#170

eg:

module "my_module" {
  source = "gcs::https://www.googleapis.com/storage/v1/my/path/module.tar.gz"
  name  = "foo"
}

[hashicorp-cla]

All committers have signed the CLA.

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