Automate applying new changes via Terraform

[bourgeoisor]

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

[bourgeoisor]

Still planning on doing this.