Genericise GitLab token fetching

The /auto-devops/release-it.yml template has some logic in it which allows retrieving the GitLab access token for driving the GitLab API. It's likely that we'll want to repeat this sort of MR automation in future so now is a good point to genericise it into something which can be !reference-d into before_script. Edit: even better, something which sets an environment variable using a dotenv report.

Ideally it'd also support fetching tokens from custom locations since https://gitlab.developers.cam.ac.uk/uis/devops/infra/gitlab-project-factory/-/merge_requests/51 adds support to the GitLab project factory to allow this.

Already we have HR Java templates doing the exact same thing as /auto-devops/release-it.yml to access tokens:

• hr/irs/panel/.gitlab-ci.yml
• hr/irs/orchestra/.gitlab-ci.yml
• hr/irs/referee/.gitlab-ci.yml
• hr/irs/irs/.gitlab-ci.yml