Add automation to check for un-labelled resources

* **Problem:** As we were working on a Jump Start Solution, we realized that we forgot to label the resources using the "labels" variable.
* See this [Google-internal doc](http://go/solutions-in-console-authoring). This document states that all Blueprint resources should set the "labels" argument (where applicable) to "var.labels".
* Ideally, we would either:
* have automation that adds these labels in any Terraform resource that supports the "label" argument.
* have automation that warns developers that Terraform resource that
* **Feature request:** Would it be possible to add a check inside the `lint` GitHub Action that warns users about unlabelled resources (for resources that support the "labels" arg)?

### Additional info
* This is a nice-to-have. Feel free to close. :)
* Here's the issue where we realized that we forgot to label resources: https://github.com/GoogleCloudPlatform/terraform-genai-knowledge-base/issues/94.