feat: add openapi docs/client generation

[Cucumberrbob]

Haven't discussed this beforehand at all, so feel free to just close this if it's not the direction you want to go in. Currently a draft until I rework the frontend to work with Problems

Just generates a client, doesn't actually use it - this PR (in order to try to be minimal) (mostly) only adds openapi document generation. Both the generated client (pretty much essential) and the generated openapi.json (not essential) have been added to git.

This is in large part useful for my sveltekit rewrite (see PR onto this branch). I can draft PR this against main on request

A few endpoints have changed:

• POST /Api/Torrents/VerifyRegex no longer accepts a file from a form The only way a form can be passed is if the body is a form (that is, body is empty). So the request must be null. So passing a file to this endpoint can never work
• Every endpoint on AuthController now returns a Problem rather than just a string

And no docs are generated for the qBittorrent compatible endpoints

Browse the generated documentation (Redocly, Swagger)