Support URL in ovirt_imageio.client.extents()

[nirs]

ovirt_imageio.client.extents() works only with filename, reporting extents in local
files.

This would be useful for debugging backups using scripts like backup_vm.py:
https://github.com/oVirt/python-ovirt-engine-sdk4/blob/main/examples/backup_vm.py

Example flow:

1. Backup tool adds --dry-run option or --dump-extents
2. Backup tool starts a backup
3. Backup tool dump extents using client.extents
4. Backup tool performs a backup or stop the backup

Possible solution

1. Support both URL ("https://...", "file:///filename") and filename ("filename") in the filename argument.
2. Parse the URL - should work for both filename and URL, if not normalize the filename to file: URL
3. Instead of _open_nbd, use _open_http for http url, or open_nbd for file: url.

This code can be used later by client.upload() to support upload from URL (qemu-nbd already supports this). More work is required for upload from URL like setting timeouts.

[nirs]

@dupondje, maybe you would like to work on this?