Support billing_project in google_project_service

[rileykarson]

billing_project is a configurable project used in user project overrides (eg https://registry.terraform.io/providers/hashicorp/google/latest/docs/guides/provider_reference#user_project_override)

It works automatically for generated resources, but doesn't work on handwritten resources where it's not manually added. We should support it here.

This is useful particularly for this resource because service activations rely on the actor (credentials) having the serviceusage.googleapis.com service enabled. This is always fine for a user account, but there's a bit of a bootstrapping problem for service accounts.

If I create project foo with my user credentials (or a privileged SA) and then create service account bar within that project, I can't later enable services inside the account because bar's project doesn't have serviceusage enabled. By setting a project override for that request, the override would be checked instead of the project of the service account. Currently-implemented, only the "direct" project (the project where the service is enabled) can be used as an override.

This may not work for all users- bar would required a permission on the billing_project. From https://registry.terraform.io/providers/hashicorp/google/latest/docs/guides/provider_reference:

When [user_project_override is] set to true, the caller must have serviceusage.services.use permission on the resource [/ billing] project.

[rileykarson]

GoogleCloudPlatform/magic-modules#5215

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