Add a generic package manager artifact type that uses the underlying OS' package manager

[aszs]

Add a generic package manager artifact type (ie. an artifact.OSPackage) to artifacts.yaml that uses the underlying OS' package manager to install a package. The artifact's implementation can detect its host's package manager if the operation_host is of type unfurl.nodes.Localhost, as it will then have a package_manager attribute.

To handle the case where the name of the package varies from package manager to package managed define optional properties that name the package for each supported package manager and prefer that name if defined over the default name (e.g. property names like apt, yum, brew)