Add fontScale property to mobile_context

[dapids]

I'd like to add information about the font scale to mobile_context.

Useful resources:

• Font size on Android
• Font size on iOS
• Current mobile_context schema

The idea is to add the following property to the mobile_context:

"fontScale": {
  "type": ["number", "null"],
  "minimum": 0,
  "maximum": 1000,
  "description": "Scale factor used to adjust the size of a font relative to a baseline size. It leverages [UIFont preferredFontForTextStyle:UIFontTextStyleBody].pointSize on iOS and getResources().getConfiguration().fontScale on Android."
}