Enhancement: Allow usage of deprecated S3 `ListObjects` API

Some S3 API implementations, e.g. Ceph before version 14.2.5, have a broken
`ListObjectsV2` implementation which causes problems for restic when using
their API endpoints. When a broken server implementation is used, restic prints
errors similar to the following:

    List() returned error: Truncated response should have continuation token set

As a temporary workaround, restic now allows using the older `ListObjects`
endpoint by setting the `s3.list-objects-v1` extended option, for instance:

    restic -o s3.list-objects-v1=true snapshots

Please note that this option may be removed in future versions of restic.

https://github.com/restic/restic/issues/3083
https://github.com/restic/restic/pull/3085
