Allow for generating a "model-only" crate #1370
Description
Currently, both the client and the server only generate code for shapes that are within the service closure.
A strength of Smithy is how easy it is to import and compose shapes from other models. Promoting shape reuse across projects is arguably a good thing; think about the many ubiquitous models (ASIN, list ID, SKU) that would be redefined with slight inconsistencies accross services and other applications.
It'd be great if we could offer, either as a configuration option for either the client or the server plugins, or as a separate plugin (or both), a way to generate shapes that are outside the closure of a service shape. For example, users could generate a model Rust crate that they could share across projects.