Describe large upload use case

[tfpauly]

Currently, OHTTP has no maximum size, so a server that is willing to accept large requests is forced to buffer unbounded messages before knowing if they can be decrypted, or if they will be served. Chunking allows the server to validate the HPKE encryption and request headers in the first chunk before accepting the rest of the request. Chunking also allows endpoints to write data out after decrypting, instead of holding the entire message in memory.