Add a generic fragment to retrieve Google Secret Manager secrets.

This MR proposes a way to retrieve Google Secret Manager secrets and export them as environment variables without the need to store the secret values as CI job artefacts.

Here is an example of the release-it pipeline working with the proposed changes - https://gitlab.developers.cam.ac.uk/uis/devops/experiments/rk725-demo/webapp-test/-/jobs/1473948

Resolves #60 (closed)