Automate applying new changes via Terraform

Currently, any changes that are applied to the `terraform/` directory (e.g. new Cloud Build triggers) need to be applied manually (via `terraform init` and `terraform apply`). We should look into automating this (e.g. a GitHub Action that runs when that directory is modified on the `main` branch which would apply Terraform changes.

Docs: https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/tree/main/terraform