feat(cli/push): Push with name if available (+ CLI flag)

[xdoardo]

As of now, while users can push a package to a namespace without a version, they can't attach to it a name as well. This patch introduces the necessary infrastructure for the CLI to be able to push unversioned (as in, named, but without a version) packages to the registry.

For example, if a manifest such as the following

[package]
name = "my-namespace/my-package"
<no version> 
...

is found in the directory of the package to push, then the result of wasmer package push is to push to the registry a package bound to the identifier my-namespace/my-package but without a version. This can behaviour can also be enforced with the corresponding --name flag in the push subcommand.

This does not allow users to run unversioned packages, as no consensus on naming was reached yet - this means that, as of now, executing wasmer run my-namespace/my-package:<hash> will fail. Instead, users can run their package with wasmer run <hash>.