Data source for IAM Testable Permissions

[modular-magician]

Release Note Template for Downstream PRs (will be copied)

Add google_iam_testable_permissions data source

Fixes: hashicorp/terraform-provider-google#6120
Fixes: hashicorp/terraform-provider-google#4812

Adds a new data source for IAM testable permissions.

data "google_iam_testable_permissions" "perms" {
	full_resource_name   = "//cloudresourcemanager.googleapis.com/projects/my-project"
        custom_support_level = "NOT_SUPPORTED"
        stages               = ["GA", "BETA"]
}

output "perms" {
	value = data.google_iam_testable_permissions.perms.permissions
}

Then the output will look like this:

$ terraform apply
data.google_iam_testable_permissions.perms: Refreshing state...

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

perms = [
  {
    "api_disabled" = false
    "custom_support_level" = "NOT_SUPPORTED"
    "name" = "appengine.runtimes.actAsAdmin"
    "stage" = "GA"
    "title" = ""
  },
  {
    "api_disabled" = false
    "custom_support_level" = "NOT_SUPPORTED"
    "name" = "cloudsql.sslCerts.createEphemeral"
    "stage" = "GA"
    "title" = ""
  },

Derived from GoogleCloudPlatform/magic-modules#3460