[DRAFT] Add image endpoints

[jellejurre]

Note: Currently a draft because it can only be merged once all generators are confirmed to work with it. My current edits to the generators assume files are .png.

Generators:

• [ ] CSharp: modified at https://github.com/vrchatapi/vrchatapi-csharp/pull/25.
• [x] Rust: modified at https://github.com/vrchatapi/vrchatapi-rust/pull/30 (merged).
• [ ] Javascript: modified at https://github.com/vrchatapi/vrchatapi-javascript/pull/12.
• [x] Java: checked, works out of the box.
• [x] Python: checked, works out of the box.
• [ ] Dart: modified at https://github.com/vrchatapi/vrchatapi-dart/pull/26.
• [x] Elixir: hasn't gotten updates in months, I don't consider this an active repo.

[Rexios80]

This generates fine in dart: https://github.com/vrchatapi/vrchatapi-dart/pull/26

[jellejurre]

This generates fine in dart: vrchatapi/vrchatapi-dart#26

It generates fine in all I've tested so far, but the multipart file type is either not supported at all or the mimetype is set wrong for it, so we'd have to test that either way

[jellejurre]

fixes: #254 #76