[Feature Request] use pre-signed URLs to upload directly to the bucket (work around 413 errors)

[Krzysztof Nazarewski]

Google Cloud Run imposes limit of 32MB requests when using HTTP/1 interfering heavily with Attic uploads.

The solution would be to use pre-signed uploads directly to the bucket.

It would possibly interfere with require-proof-of-possession.

[Cole Helbling]

I've put up a WIP branch here that does this: https://github.com/cole-h/attic/tree/presign-uploads

Could you try that out and see if it behaves how you would expect?