Enhancement: Allow usage of deprecated S3 ListObjectsV1 API

Restic didn't work with some S3 API implementations that don't have a
broken `ListObjectsV2` API endpoint. This concerns mainly Ceph versions
before v14.2.5 , but may impact other S3 API implementations. 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 selection of the older
`ListObjects` endpoint by using the `s3.list-objects-v1` extended option, for
instance:

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

This option may be removed in future versions of restic.

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