add audio.network `url_callback` and `headers_callback`

[cuifengcn]

I'm using Playlist(audio: network_audios) to open songs, but every song url is variable, i should get the right url before play it, so url_callback and headers_callback could be very helpful.

Audio audio1 = Audio.network(..., url_callback: xxx, headers_callback: xxx);
Audio audio2 = Audio.network(...);
audioPlayer.open(Playlist(audios: audios));