support sbom/attestation OCI artifacts similar to .sig signatures

**Describe the problem/challenge you have**

we currently have --cosign-singatures=bool flag that attaches .sig artifacts. cosign has .sbom and .att suffixes as well.
(https://github.com/sigstore/cosign/blob/0142711da2fadc78f546a99adf12e2f0be428600/pkg/oci/remote/options.go#L27-L29)

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