Support custom duration formatter

[MarsWaveX]

This is an issue to describe what https://github.com/suragch/audio_video_progress_bar/pull/80 does and why we need it.

Sometimes we need more control of how we format the duration, for example, we want format duration to "01:01" style.

And when playing the audio which can not fetch the total duration, like HLS stream which is still being generated, the "total" duration will be "Duration.zero". If we support custom duration formatter, we can give ProgressBar a fake total duration, then format it to "--:--", the UI will be more user-friendly.

The HLS stream audio behind this screenshot is playing.