s3store: Allow customizing upload ID, object ID and bucket

[Acconut]

Previously, the upload ID included the multipart ID, which made fully customizing the upload ID (and thus upload URL) impossible. With this PR, we hope to allow full customization of the upload ID and even the destination object key.

Overall changes:

• Removes the multipart ID from the upload ID and instead loads it from the info file
• Allow customization of upload ID
• Allow customization of destination object key via Storage.Key
• Allow customization of destination bucket via Storage.Bucket

TODO:

• Manual testing