Add CI test job to check specs are up to date

Adds a latest-spec test job that uses ./pull-specs.sh to get the latest spec files and fails if this causes a diff from what is there.

Needs an access token to pull from other repos, so a group access token has been created on the /uis/devops/iam folder and added as a CI var GITLAB_PULL_SPECS_TOKEN in this project.

https://gitlab.developers.cam.ac.uk/uis/devops/iam/identity-lib/-/jobs/692201 shows the failure as the specs are actually out of date

Will need to update the specs - needs coordination

Rebased with up to date specs and pipeline succeeds: https://gitlab.developers.cam.ac.uk/uis/devops/iam/identity-lib/-/jobs/693593

Closes: #14 (closed)