Update pull-openapi-specs script to allow for pinning to commit

Related to: https://gitlab.developers.cam.ac.uk/uis/devops/iam/card-database/card-api/-/issues/71

This change allows us to optionally pin our API specs to specific commit shas, which allows us to make other updates to the identity-lib while we iron out large changes in the card API specification update from 2.0 to 3.0.