Support `File` uploads

[sobolevn]

This is a feature for 0.2.0 release, we can start without it.

We need to design file uploads.
Sources of inspiration:

• https://docs.djangoproject.com/en/6.0/topics/http/file-uploads/
• https://docs.litestar.dev/latest/usage/requests.html#file-uploads
• https://www.django-rest-framework.org/api-guide/parsers/#fileuploadparser
• https://django-ninja.dev/guides/input/file-params/

Key points:

• File uploads must follow the regular path that Django's file uploads do
• They should be customizable via BACKENDS, which is done by Django by default
• They should be type safe
• Schema generation must pass schemathesis

[sobolevn]

Nope, I rescedule it to be 0.1.0, because we have several important design problems that File helped to identify.

[sobolevn]

Docs: https://swagger.io/docs/specification/v3_0/describing-request-body/file-upload/

Here's how I think it should be:

1. We need to handle different independent single file uploads with different content-types, like image/png
2. We need to handle file uploads with multipart/form-data and multiple fields, possibly via new component called Form
3. We need to be able to send one big json field in multipart/form-data and be able to load it as request's Body like in https://django-ninja.dev/guides/input/file-params/#uploading-files-with-extra-fields
4. We need to provide a middleware to fix file uploads to non-POST methods in django, the same way django-ninja does: https://github.com/vitalik/django-ninja/blob/master/ninja/compatibility/files.py
5. We need to provide a default model for file validation. We need to be able to validate: filenames, file types, size in binary

Things to validate:

• File[list[FileInfo]] - any number of files
• File[tuple[FileInfo, FileInfo]] - exactly two files
• File[FileInfo] - exactly one file
• File[FileInfo | None] - one or zero files

Extras:

• Parser types for files and multipart data
• Smart validation of component_parser, so Body cannot be used with File in the same controller