Add upload endpoint

[atrawog]

This PR adds the api endpoint /channels/{channel_name}/upload/{filename} to quetz. The new endpoint includes a mandatory sha256 checksum verification and uses an async request.stream that is calculating the sha256 checksum while uploading.

An example file upload for sqlite-3.37.2-hc67169a_0.tar.bz2 with curl using the new endpoint looks like this:

curl -i -H  "X-API-Key: ${QUETZ_API_KEY}" \
-X POST http://127.0.0.1:8000/api/channels/uptest/upload/sqlite-3.37.2-hc67169a_0.tar.bz2?sha256=63da44f64fb627ad144d38df48b24fb741f8f787027b38219273b40ac75bada9 \
--data-binary @sqlite-3.37.2-hc67169a_0.tar.bz2

[wolfv]

This looks good to me. The only modification I would like is to also expect either a Content-Length header or a ?size=... argument to have knowledge about the expected file size. While this doesn't give any additional guarantees vs. the hash, I think it could be quite useful to later e.g. abort an upload early if not enough space is available and so on.