Add object storage sink for weekly bundles

Allegedly can bundle historical PLC operations into weekly compressed archives. Right now it has a file writer for that, but ideally it would also be able to write directly to S3-compatible object storage!

The file-writing bit is here: https://tangled.org/@microcosm.blue/Allegedly/blob/main/src/weekly.rs#L127-177

An S3-writing version should look pretty similar. The https://lib.rs/crates/object_store crate appears to support both local file writing and object-storage, which might be perfect.