Add Service Account for Accessing Secure Endpoint

[madhavajay]

We need to add a service account or equivalent secure connection between the "permission validator" cloud function endpoint and the Permission Portal backend code.

Currently the old end point has been set to internal access only:
https://us-central1-covidwatch-354ce.cloudfunctions.net/createPermissionNumber

We need to ensure that this function can be invoked but only from the Permission Portal.