Update transport module to be public

[marsella]

We want to reuse the zeekoe transport module in the key-mgmt product, but it's currently private.

Easiest solution: make it a public module within zeekoe.
Another option: make it its own crate.

In either case, browse through the module before just making it public to be sure that the public interface is appropriately scoped.

[indomitableSwan]

I would advocate for modularity, i.e., splitting out any zeekoe logic we want to use into its own crate.

[marsella]

I think this is closed with #366 and the creation of the transport crate. Can re-open if we realize this isn't complete.