Draft: Backport 3.0.x: !3023: lua: playlist: add a way to change playlist meta

Expose a way to change playlist metadata in Lua. For now add a specific call to the vlc interface only available in the parse function, it should be used that way:

function parse()
    -- This is the usual "meta" Table also used on returned items.
    vlc.set_current_playlist_meta({
        title="Podcast title",
        arturl="https://artworkuri.jpg",
    })

    -- ...
end