Use multer instead of axum's multipart

[jan-auer]

Axum has a multipart extractor that opaquely wraps multer::Multipart. It lacks two essential APIs:

• Options cannot be configured, such as size limits for individual items. We require these limits and currently have implemented a hack to enforce them.
• The error is opaque, but we would like to inspect and react to the variants.

Since we already use multer for parsing nested multipart bodies, we can remove the multipart feature from axum and implement a simple extractor wrapper ourselves.