Add dafny-types-codgen SmithyBuildPlugin

[robin-aws]

This would live inside the same smithy-dafny-codegen library specified in #151, but would be a separate plugin from dafny-codegen.

The general idea for this kind of Smithy plugin is that generating the types for a set of Smithy shapes has use cases outside of client or server code generation. This is particularly true for Dafny, where just being able to convert between the generated types for Dafny and one or more target languages in-memory can be valuable.

Needs more design elaboration. At the time of writing this, no other smithy-<language> tool implements this plugin. It is only hinted at in the Smithy specification: https://smithy.io/2.0/guides/building-codegen/configuring-the-generator.html#how-to-name-codegen-plugins

[robin-aws]

Specification is evolving quickly to fill in details on this use case, and the shapes property of services is about to land: https://smithy.io/2.0/guides/building-codegen/overview-and-concepts.html#type-generation