Add support for GCP IAM impersonation

[antoineverin]

Back port of PR: cyrilgdn#448

Add support for GCP IAM service account impersonation

Use cases

The company has a centralized service account that is used for Terraform automation. However, such GSA should not be used to access the database directly where each database will have its own IAM DB users.

This added an option to impersonate the database IAM user via the centralized GSA. As long as the centralized GSA has sufficient permissions to impersonate as the database IAM DB user, it can be used to perform database automation in Terraform.

Testing

resource "google_sql_database_instance" "self" {}
resource "google_sql_user" "admin" {}
resource "google_service_account" "db_iam_admin" {}
resource "google_sql_user" "iam_admin" {
  name     = trimsuffix(google_service_account.db_iam_admin.email, ".gserviceaccount.com")
  instance = google_sql_database_instance.self.name
  type     = "CLOUD_IAM_SERVICE_ACCOUNT"
}
resource "google_project_iam_member" "iam_admin_project_iam_members" {
  for_each = toset(["roles/cloudsql.client", "roles/cloudsql.instanceUser"])
  member   = google_service_account.db_iam_admin.member
  role     = each.key
}

provider "postgresql" {
  scheme                              = "gcppostgres"
  host                                = google_sql_database_instance.self.connection_name
  username                            = trimsuffix(google_service_account.db_iam_admin.email, ".gserviceaccount.com")
  gcp_iam_impersonate_service_account = google_service_account.db_iam_admin.email
  port                                = 5432
  superuser                           = false
  alias                               = "iamAdmin"
}

# it should work and able to apply resources using the IAM db user
resource "postgresql_*" "*" {
  provider = postgresql.iamAdmin

  // *
}