feat: add detect-non-utf-files job

Closes #134 (closed)

Examples:

• File with utf-16 detected in the repo: https://gitlab.developers.cam.ac.uk/uis/devops/experiments/du228-testing/du228-utf-encoding-test/-/jobs/2950272

• File is still there, but added in allow list: https://gitlab.developers.cam.ac.uk/uis/devops/experiments/du228-testing/du228-utf-encoding-test/-/jobs/2941888