Add universe domain and custom endpoint support to Packer

[cdunbar13]

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Description

In the other Terraform providers we can add in the universe_domain and custom_<service>_endpoint fields to specify other uris for the API endpoints and entire other sets of infrastructure. Packer's endpoints seem to be hard coded, and I would like to see this exposed to the user.

Use Case(s)

Users of GCP Trusted partner clouds (TPCs) may want to build images using packer, but they would not be able to because they would be in a different universe.

Additionally this would be useful for internal testing where we use different endpoints for APIs that haven't become available yet.