Ensure that the networksecurity p4sa is permissioned in tests

[modular-magician]

Implements the guidance from https://cloud.google.com/secure-web-proxy/docs/enable-tls-inspection#create-service-account. This could technically be done at environment bootstrap time, but the identity resource is idempotent and the permission is tied to the specific pool, so let's just do it inline.

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Derived from GoogleCloudPlatform/magic-modules#8222