feat: add OpenAPI generator template

As part of [the releated Activate Account epic][1] a generic template
was created to allow OpenAPI clients to be generated from YAML schemata.

#93 covers moving this
template to the ci-templates repo and documenting it in the guidebook.
This provides the ci-template-side part of that issue.

The actual content is a straight copy-paste from the upstream template
and so should require minimal review.

[1]: uis/devops&205
[2]: https://gitlab.developers.cam.ac.uk/uis/devops/iam/activate-account/api/-/blob/main/.gitlab/openapi-generator.gitlab-ci.yml?ref_type=heads