Sign a batch of uploads at once

When preparing a large number of uploads in a workspace, all uploads will need to be signed to publish to the main repository. But our CLI can only sign a single workflow at a time.

We could support something like debusine work-request sign --all -l *.changes to sign everything we can find, locally.

Or just have a debusine work-request sign --all that interactively lists each work-request that's waiting for signing and signs it.

This functionality would probably benefit from filtering in work request list API (#1181).