Use smithy-rs to generate shapes for Rust

[robin-aws]

Ideally we should be able to take a dependency on the smithy-rs plugins and use it to generate most of the types for the Rust interface to a polymorph library (see https://github.com/smithy-lang/smithy-dafny/blob/main-1.x/TestModels/SimpleTypes/SimpleString/runtimes/rust/README.md)

It sounds like we can likely provide some kind of "no-op protocol" as a plugin and omit most of the code that isn't relevant.

Depends on smithy-lang/smithy-rs#421 since the smithy-rs plugins aren't published to Maven Central yet. For now we can use a submodule and local maven publishing.

[robin-aws]

This is more of an implementation idea than a task to complete - closing in favor of concrete deliverables such as #375