Code generator version number should be emitted in generated crate

[drganjoo]

Motivation and Context

Looking at the code that was generated by the "SmithyRs Code Generator" one cannot tell the version of the generator that was used.

Knowing the versions can help in post-deployment to track version specific bugs, and to potentially inform customers to regenerate in case of some vulnerability.

What is required

Cargo.toml has a metadata field that can be used to write additional information about the generator.