Add WebVTT Mp4 To Mp4 Support

[vaage]

The packager does not support WebVTT MP4 input and WebVtt MP4 output. The WebVtt in MP4 output code is usable as it, but the WebVtt in MP4 input code won't pair-up with the output code as is. The problem is that the input media handler produces Media Samples but we need text samples in order for the rest of the pipeline to work.