Add acceptance tests for how provider handles billing_project arguments

[modular-magician]

This PR adds acceptance tests for usage of billing_project that demonstrate:

• how the provider behaves when provider configuration arguments come from different sources ( config vs ENVs)
• schema-level validation that's in place, e.g. handling of empty arrays
• use cases: how does this argument impact the providers behaviour in plan/apply
  • I've tested the use of billing_project + user_project_override=false to show that billing_project has no impact then
  • I've tested the use of billing_project + user_project_override=true to show that billing_project does have impact then

NOTE: I added some usage tests for the PF provider, but they should be replaced in future because being limited to Firebase data sources to test the PF provider makes the test very awkward

Release Note Template for Downstream PRs (will be copied)

Derived from GoogleCloudPlatform/magic-modules#11610