feat(integration): Integrate Chainloop in a Sigstore environment

[jiparis]

The goal is to research the feasibility of integrating Chainloop and Sigstore by:

• Letting users use a Fulcio instance to generate ephemeral signing certificates with the proper attributes (CTlog inclusion, OIDC attributes correctly mapped, etc)
• Providing integration with Rekor to publish signatures in a public/private transparency log
• generating attestation bundles in the Sigstore Bundle format (the one GitHub uses) to store the verification material
• Ensuring the attestations are correctly signed and can be verified using the Sigstore verification specs (implemented in the sigstore-go library)

The outcome of this task would be a set of action items to tackle as part of this initiative.

### Tasks
- [ ] https://github.com/chainloop-dev/chainloop/issues/1244