git-lfs-transfer: Enable in shell and introduce batch download

As documented at https://github.com/git-lfs/git-lfs/blob/main/docs/proposals/ssh_adapter.md#requests-to-transfer-objects, the download operation handles sending objects to the client.

Once the download operation is implemented, the get-object command needs to be implemented as described at https://github.com/git-lfs/git-lfs/blob/main/docs/proposals/ssh_adapter.md#downloads which is being handled with git-lfs-transfer: Implement get-object (#753 - closed).