introduce dvc ls-url

[efiop]

We have get/import/ls for dvc repos, but only get-url and import-url for clouds. With cloud workflows in mind, we need dvc ls-url to be able to list files on clouds (including being able to list available version_ids for cloud versioning workflow in the future). This will also be useful for get-url/import-url to be able to discover files before downloading/importing them.

This is also a nice benchmark(-ish) for comparing dvc get-url/ls-url with other tools like awscli, rclone, etc.