Generate API models with openapi-generator plugin in new module

[bherbst]

Summary:
Our API models are woefully out of date, and unfortunately I have not had success regenerating our models using the existing infrastructure.

The existing tooling relies on a fork of swagger-codegen that generates interfaces instead of concrete Java classes.

This fork was made when the latest version of swagger specs was 2.0, but TBA's API specs now leverage OpenAPI spec version 3.0.1.

In theory we could bring our swagger-codegen fork up to date with upstream, however there are a few reasons I'm not pursuing that:

• Leveraging a published tool as-is will allow us to more easily update to new versions in the future
• I don't personally believe we get much value in generating interfaces over concrete classes. In fact I believe this adds significant additional complexity and we could get a lot more "for free" such as not needing custom deserializers if we lean in to out-of-the-box functionality

This PR only creates a new tba-api module with models and Retrofit interfaces generated by openapi-generator. This PR does not change any existing code to leverage the newly generated modules, and we will need to work our way through the existing codebase to migrate. I plan to do this in small chunks, roughly one at a time as possible.