Backup repository direct-to-object storage RPC

Create a RPC that uploads a full repository backup (bundle, refs, hooks) to object storage. Later this RPC will be used by praefect in order to coordinate backups with gitaly replicas.

Initially it is expected that the directory structure will match that of gitaly-backup. See https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/gitaly-backup.md#pointer-layout