Add new GCP Reference Architecture Terraform Module

Relates https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit/-/issues/22 Closes https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit/-/issues/23

Much like our new AWS implementation we want to add in a new Terraform module that streamlines the config needed to set up a full GCP Reference Architecture. Currently users need to add a substantial amount of config for each component individually that is the same for each environment. The new module builds on top of the current one and abstracts away the standard code to reduce repetition.