Add GSM Terraform module to dogfood

Created by: ElizabethStirling

We need to enable / configure GSM for our sourcegraph-dogfood project and deploy Berglas infrastructure to the GKE cluster. Note that this should undergo a lot of testing, since mutating webhooks are problematic if misconfigured.

Dependencies: #17793 (closed)

Useful Links: Example Terraform module Using Terraform modules

Repository: Infrastructure