Added GitLab CI/CD template for running Diffblue Cover

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/137047



Merged-by: Avielle Wolfe <awolfe@gitlab.com>
Approved-by: Avielle Wolfe <awolfe@gitlab.com>
Reviewed-by: Thong Kuah <tkuah@gitlab.com>
Reviewed-by: Nick Malcolm <nmalcolm@gitlab.com>
Co-authored-by: Rob Oxspring <rob.oxspring@diffblue.com>