feat(vault): Add example secret to mvp vcluster + README

Context

• Minimal example of making a Vault secret accessible in a nomad job (OpenProject task 1036)
• Any security implications are out of scope for this example (followed up in task 1556)
• Depends on vs-example!6 (merged)
  • This MR is in draft until vs-example!6 (merged) is merged and the hash is updated here

Impact

• Added manual steps to the README for running the vcluster mvp example with a secret

Test(s)

• Create the vc-shared-services base infrastructure
• Make sure to test this branch with the vs-example branch from vs-example!6 (merged) ❗
• Follow the Readme in this MR to set up port forwarding + secret as needed
• After terraform apply, log in to Nomad and check the logs for the echo job. The example secret should be printed there:

Known issues

• It is a manual step to add a Vault secret
• For production, this example needs to be followed up with a more secure version
  • No secrets printing in cleartext
  • Ensure HTTPS for vault access
  • The secret should be sensitive inside Terraform
  • To be checked if the Vault token or secret are logged or stored somewhere in Terraform
  • To be checked if the secret is handled securely besides the above points
• The security issues are followed up in task 1556

Links

• OpenProject task 1036