list crates to be published

[marcoieni]

Add a way to list crates to be published, for example cargo ws publish --list.
This will not publish the crates. Instead it will just list the crates that should be published.

I know there is cargo ws changed, but it checks git tags, not the cargo registry.

With this feature you can:

• use external tools to publish the repositories, for example cloudsmith-cli.
• have a quick look to what crates need to be published.
• pipe the result in other tools to get other functionalities (for example to generate changelog).

[pksunkara]

Added cargo ws plan subcommand for this.