Add set_label / unset_label to the DEB PackageViewSet policy #1504

The DEB PackageViewSet policy permits only list, retrieve, and create. Therefore, set_label and unset_label return HTTP 403 even for users with core.manage_content_labels.

RPM package endpoints already allow these actions. Adding equivalent DEB policy support, gated by core.manage_content_labels, with permission tests.

This is required to add labels to existing DEB content through the Pulp API without modifying the Pulp database directly.