Add support for "loading" attribute to FileMediaFormatterBase

Problem/Motivation

Chrome now supports lazy loading on video and audio elements: https://caniuse.com/loading-lazy-media
This would avoid using bandwidth if the user does not scroll to the video/audio player.

It can also be combined with the preload attribute (#3048458: Add support of audio/video "preload" attribute into FileMediaFormatterBase) to control what is preloaded when the lazy loading is triggerd.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet