change way tracks are initialized

A config object is now required to initialize tracks.
Config objects are provided for H264 and AAC.
This allows to pass parameters easily and treat AAC
parameters explicitly.

examples/client-publish-options/main.go

@@ -36,7 +36,7 @@ func main() {
 	fmt.Println("stream connected")
 
 	// create a H264 track
-	track, err := gortsplib.NewTrackH264(96, sps, pps)
+	track, err := gortsplib.NewTrackH264(96, &gortsplib.TrackConfigH264{sps, pps})
 	if err != nil {
 		panic(err)
 	}