Provide options for pushing and pulling from OCI registries

[radu-matei]

https://github.com/engineerd/signy/blob/d648918f7c6dadf8ddb4a15d71a6db86a323d02b/pkg/cnab/push.go#L18

For example, the function that exposes the cnab-to-oci Push functionality only allows the target reference and bundle file to be passed, as opposed to the wider range of options allowed by cnab-to-oci.

https://github.com/docker/cnab-to-oci/blob/bf0672a3966c7a41fb7ffb524231d79cb5e16f31/remotes/push.go#L36

While not all options might be relevant for the signature, some might be - for example passing a relocation map.