Add final Terraform script for GC runner provisioning

What does this MR do and why?

This MR add the final Terraform script for GC runner provisioning. It moves the content into an erb template, and adds some validation on arguments, to prevent any injection attempts, as in the future these scripts may get executed automatically.

EE: true

Part of #438316 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Run the following query in http://gdk.test:3000/-/graphql-explorer:

   mutation provisionGoogleCloudRunner($input: ProvisionGoogleCloudRunnerInput!) {
     provisionGoogleCloudRunner(input: $input) {
       provisioningSteps {
         title
         languageIdentifier
         instructions
       }
       errors
     }
   }

   Variables:

   {
     "input": {
       "dryRun": true,
       "projectPath": "gitlab-org/playground",
       "provisioningProjectId": "dev-gcp-s3c-integrati-9abafed1",
       "ephemeralMachineType": "n2d-standard-2",
       "provisioningRegion": "us-central1",
       "provisioningZone": "us-central1-a",
       "runnerToken": "glrt-Ei-AAaaaaAAAaaAA"
     }
   }