Draft: feat: added GCP secret manager support using Workload Identity Federation.

Closes #60 Awaiting new functionality in GitLab stops anyone viewing the artifacts. This will mean we can use dotenv functionality to return secrets without the risk of the secret being exposed in public repos.

Documentation artifactsaccess suggests this was live as in 16.10, but didn't appear to be available when I tested it.

MR 145206 suggests this may be in 16.11 instead.

artifacts:
  access: none

In the meantime, I have a test harness on this branch showing how to use:

• test-gcp-secret-manager-template