Add new experimental-image-proxy hidden command

This imports the code from https://github.com/cgwalters/container-image-proxy

First, assume one is operating on a codebase that isn't Go, but wants
to interact with container images - we can't just include the Go containers/image
library.

The primary intended use case of this is for things like
[ostree-containers](ostreedev/ostree-rs-ext#18)
where we're using container images to encapsulate host operating system
updates, but we don't want to involve the [containers/image](github.com/containers/image/)
storage layer.

Vendoring the containers/image stack in another project is a large lift; the stripped
binary for this proxy standalone weighs in at 16M (I'm sure the lack
of LTO and the overall simplicity of the Go compiler is a large factor).
Anyways, I'd like to avoid shipping another copy.

This command is marked as experimental, and hidden.  The goal is
just to use it from the ostree stack for now, ideally shipping at least
in CentOS 9 Stream relatively soon.   We can (and IMO should)
change and improve it later.

A lot more discussion in cgwalters/container-image-proxy#1

cmd/skopeo/main.go

@@ -84,6 +84,7 @@ func createApp() (*cobra.Command, *globalOptions) {
 		copyCmd(&opts),
 		deleteCmd(&opts),
 		inspectCmd(&opts),
+		proxyCmd(&opts),
 		layersCmd(&opts),
 		loginCmd(&opts),
 		logoutCmd(&opts),