OpenAPI export functionality

Use the information from routing, validation middleware, and possibly typescript return types[1] to generate Swagger YAML for ingestion into various tools.

The goal is to be able to feed Boltzmann-generated OpenAPI documents into various documentation generators & request tools (like Postman.) A stretch goal is that we can _lint_ these API docs.

[1]: We could also expose a `validate.returns({...jsonschema})` middleware that does response validation on the way out in dev mode for non-typescript projects? IDK exactly.