Switch to OpenAPI

[qwerty01]

The new beatsaver has an OpenAPI specification. I've tried it out a little already and it's mostly correct but needs a few changes, such as removing the Tag enum, and a lot of required fields are marked as optional, such as map version, map version hash, etc.

The rust openapi generator generates all the api calls too, so this would limit us to tokio as a runtime instead of supporting async-std too.

[qwerty01]

I'm not 100% sure when I'll get around to writing a user-friendly wrapper around the openapi interface, but I set up a generated openapi repository at beatsaver-openapi