Implement oci push

[allisonkarlitskaya]

This isn't supported by containers-image-proxy-rs. Options:

• implement it in containers-image-proxy-rs
• shell out to skopeo
• use https://crates.io/crates/oci-distribution

Although implementing it in the image proxy seems like the obvious approach (since we already use it) Colin has raised some doubts about the longevity of this project and says he prefers the "shell out to skopeo" approach for the time being.