(#23403) 'autosign' setting should manage file parameters

Puppet settings of type :file can specify the owner, group, and mode of
the file specified by the setting, like so:

    [master]
    autosign = $confdir/autosign.conf { mode = 664, owner = puppet, group = puppet }

Commit bca628dc extracted the autosign setting validation to a setting
class but dropped the ability for file settings to be specified. This
commit resolves the issue by making the autosign setting inherit from
the file setting.

This is targeted at stable, but should be merged either if we do a 3.4.0-RC3, or in 3.4.1 if we hit 3.4.0 final.