Backports class doesn't support sources format

[thka2315]

Describe the Bug

When loading backports class it always create the repo file in the old format and there is no way to change it.

Expected Behavior

Have an option to set sources_format to 'sources' and the repo file is created in the new sources format.

Steps to Reproduce

Steps to reproduce the behavior:

1. include apt::backports

Environment

• Version 10.0.1
• Platform Ubuntu 20.04

Additional Context

My suggestion is to add "Enum['list', 'sources'] $source_format = 'list'," to backports.pp's parameters and
add "sources_format => $sources_format," to the apt::source call in the end of backports.pp.