Extract OpenAPI docs into a "Signify" protocol repo

[m00sey]

No description provided.

[lenkan]

I have noticed currently that the API docs comments are a bit outdated. So we probably have to clean it up first. Also want to add that if we do create proper request and response type schemas, we can probably generate most of the type information needed in signify clients, instead of having to write them by hand. I have done a lot of client code generation based on OpenAPI specs before and it usually works quite well.