Add user project overrides for Cloud Functions

[danawillow]

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Description

In #5388, we learned that the Cloud Functions API is no longer accepting gcloud credentials for waiting on Operations, which leads to a not-so-great UX for creating or modifying those functions from Terraform (the function ends up getting created correctly, but polling fails, so future TF runs won't realize it was created correctly).

Users can work around this for generated resource by using User Project Overrides, but we don't have that support yet across the board for handwritten resources; it has to be added one by one. We can do this by adding a https://godoc.org/google.golang.org/api/cloudfunctions/v1#OperationsGetCall.Header to the call setting X-Goog-User-Project like we do in sendRequestWithTimeout (and a few other places). Just needs some plumbing to pass along the project through and to check whether the user is using user project overrides.

New or Affected Resource(s)

• google_cloudfunctions_function

References

• b/148479480
• b/274819432