Use GitLab.com as an OIDC provider for cosign
## Overview This epic tracks work related to making [`cosign`](https://github.com/sigstore/cosign) automatically use GitLab.com as an OIDC provider when used in GitLab CI. See [diagram](https://gitlab.com/gitlab-org/gitlab-runner/-/issues/29063#note_1244699554 "Sign build artifacts in Runner with Cosign") for a high-level overview of how this would work.
epic