Add support of "preload" attribute into FileMediaFormatterBase

Problem/Motivation

FileMediaFormatterBase doesn't allow to specifie preload attribute on video and audio file. This attribute allow to control media loading behavior.
(See : https://developer.mozilla.org/en-US/docs/Web/HTML/Element/video)

Proposed resolution

• Add new "preload" select field into FileMediaFormatterBase with three options :
  • None : The author thinks that the browser should NOT load the video when the page loads,
  • Auto : The author thinks that the browser should load the entire video when the page loads,
  • Metadata : The author thinks that the browser should load only metadata when the page loads.
• Default value must to be "metadata"

Remaining tasks

• Create patch to allow this
• Update tests for FileVideoFormatter and FileAudioFormatter to check preload attribute exist
• Update schema for the configuration file of the File module

User interface changes

• New field into configuration form of FileVideoFormatter and FileAudioFormatter
• Update summary of Fieldformatter

API changes

None

Data model changes

• update field.formatter.settings.file_video mapping.