Add `current_time` attribute to `Audio` widget

[gMatas]

Hi, 👋 this is my first pull request for this project.

I've added current_time attribute to the Audio widget, which allows user to set programmatically where the media should play from.

Somewhat relates to the issue #3063.

[github-actions[bot]]

:point_left: Launch a binder notebook on branch gMatas/ipywidgets/feature/audio-widget-current-time