feat(terraform): allow assuming TerraformPlanRole from github oidc role

[rochecompaan]

Summary

This PR updates GitHub OIDC IAM permissions in the Terraform base module so CI can assume TerraformPlanRole.

What Changed

• Added a new IAM policy in github-iam-role.tf (/home/roche/scaf-fullstack-temlate/template/terraform/modules/base/github-iam-
  role.tf) that allows:
  • sts:AssumeRole on arn:aws:iam::${var.account_id}:role/TerraformPlanRole
• Attached that policy to the existing GitHub OIDC role ({{ copier__project_slug }}-github-oidc-role).

Why

• Enables GitHub Actions workflows using the shared OIDC role to assume TerraformPlanRole for Terraform plan operations.

Scope / Impact

• Affects only IAM permissions for the GitHub OIDC role in the base Terraform module.
• Existing ECR push permissions remain unchanged.