Publish AchInterface file as part of the Open Api package in the main project

[savrielynck]

Hi,
I'm currently trying to integrate moov/ach (Awesome work btw for me who doesn't know much about the protocol!).
I was wondering if you considered publishing the types interface as part of the OpenAPI in order for consumer to be able to have the types directly into their client.

I'll be doing a node integration and the only solution that I've at hand is to copy/paste the AchInterface in my own project in order to generate the file and pass them to the moov/ach HTTP server for validation after words.

Took me a bit of time to crawl up the information and would save some times for the next person trying an integration!

note: A link in the main README for the Typescript type to this file would be great as well.