Ideas for major changes in Storage API v2

[alexwlchan]

This is a ticket to track any ideas for breaking changes we might like to make in a v2 API. It's not for discussing the changes or making a final decision on what we'll change, but a place to write them down until that API version eventually arrives.

• Support arbitrary URLs in the ingest request. Currently you have to supply an S3 bucket/key, which creates an annoying coupling: the storage service has to know every system it might ingest bags from, and have appropriate permissions configured. If we could support arbitrary URLs, you can do things like pre-signed S3 URLs from sources the unpacker otherwise has no knowledge of. See discussion in Slack: https://wellcome.slack.com/archives/CN56BRQ5B/p1628632092022000
• Support multiple checksums in the storage manifests. Currently the storage manifest will display a checksum, but the storage service supports reading multiple checksums from a bag. It would be useful if these were all available through the bags API.