Support keyless commit signing with GitSign
### Problem to solve
As a project maintainer, I want to be able prove that I authored a commit without having to manage cryptographic keys.
### Proposal
Create an integration with [GitSign](https://github.com/sigstore/gitsign), so that users can have their commits be signed automatically using their GitLab OIDC identity.
### Intended users
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
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