ADR 004: Bundle cosign with GitLab Rails, perform attestation in GitLab Rails backend

Why is this change being made?

We are currently planning work for the gitlab-org&15858 epic. As part of this we have performed some thorough analysis of specific locations and techniques we could use to perform SLSA attestations. In the course of this analysis, @darbyfrey and @stanhu pointed out that adding the cosign binary to the GitLab distribution is feasible, and has several advantages noted in the ADR. The purpose of the ADR is to highlight several options considered as well as note the decision to perform SLSA Attestation generation in Sidekiq workers, using the cosign command-line program.

CHANGEME

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