Configure IAM policy of an existing GCP folders.

[rosbo]

Blocked by #412

Implementation note: This looks very similar to what already exists for "google_project". However, since IAM policies can be set on many different resource types, it might make sense to revisit the current approach, maybe it can be generalized a bit so that it works for other resource types, too.

#132 is the parent task tracking all the work necessary to fully support Folders in Terraform.

[ghost]

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