Fix supported permissions behavior

[jwtracy]

Addressing this bug, https://github.com/terraform-google-modules/terraform-google-iam/issues/158

Adds a testable permission data source for the targeted entity. These supported permissions are set intersected with the included permissions of the role then set subtracted by the excluded permissions (either explicit or unsupported data source).

This might be a bug in the testable permissions API. resourcemanager.projects.list should be returned in the unsupported permissions for projects.

Note in the test below, resourcemanager.projects.list is not included in the role even after being removed as a manually excluded permission.

 # module.custom-role-project.google_project_iam_custom_role.project-custom-role[0] will be created
 + resource "google_project_iam_custom_role" "project-custom-role" {
     + deleted     = (known after apply)
     + description = "This is a project level custom role."
     + id          = (known after apply)
     + name        = (known after apply)
     + permissions = [
         + "iam.roles.create",
         + "iam.roles.delete",
         + "iam.roles.list",
         + "iam.serviceAccounts.create",
         + "iam.serviceAccounts.delete",
         + "iam.serviceAccounts.disable",
         + "iam.serviceAccounts.enable",
         + "iam.serviceAccounts.get",
         + "iam.serviceAccounts.getIamPolicy",
         + "iam.serviceAccounts.list",
         + "iam.serviceAccounts.undelete",
         + "iam.serviceAccounts.update",
       ]
     + project     = "ci-iam-aea9"
     + role_id     = "iamDeleter"
     + stage       = "GA"
     + title       = "iamDeleter"
   }

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[gorshunovr]

Hello. I'm hitting this error. Workaround: add excluded_permissions = ["resourcemanager.projects.list"] to the module parameters.

Please, reopen. Thank you.

[dirsigler]

The provided solution acutally fixes the problem. Can this please be merged?