This PR updates the implementation of the proxy configuration by using the apt::settings defined resource type instead of passing the configuration value through the apt class resource.

This allows previous inclusions of the apt class without a duplicate resource error, and achieves the same end result, including executing the apt_update exec afterwards. By checking for a previous definition of apt::setting used by the main apt class when configuring a proxy setting there, we can avoid duplicate resource errors as well.

The metadata has also been updated to include puppetlabs/apt version 9.x, since 9.0.2 is currently released and in testing works well with this module.

All existing unit tests pass successfully, and a functional test switching between the old implementation and the new revealed no effective changes in the catalog.

Alternate implementation of #11
Fixes #12