Allow attestation upload for DSSE type similar to the intoto type

[aalsabag]

As discussed in Slack, we would like to have attestations uploaded to our attestation store (s3) similar to how the intoto type behaves. This is all because of the choice of cosign to abandon the upload of the intoto type and only have the dsse type.
This implementation does not break existing functionality so I don't think it requires reversioning of the DSSE type. This also still allows prevention of attestation upload in the public rekor instance.
I've gone ahead and raised a PR, but am open to any suggestions.