support pushing attestations to multiple destinations

[ChaosInTheCRD]

Currently witness can only push attestations to one destination besides writing it to disk, which is Archivista.

It would be useful to expand this to be able to push the location to wherever the user decided it to be most worthwhile (e.g., OCI Registry). On top of this, it would be useful for the user to be able to specify multiple destinations for a witness run command to push the attestation collections that it generates to.

[colek42]

@mikhailswift and I have discussed the ability for multiple backends, i.e. Rekor entry, S3, OCI, etc.

[matglas]

One comment on Rekor. As far as I remember there is a size limit on Rekor which makes it less feasible for witness sizeble attestations. 😅