Allow reading from/writing to iOS shared app group directory

[mohammadrafigh]

Feature Request

Motivation Behind Feature

There are many cases that there is a need to access files and directories of the main app target from another target (extension).
For example, if you want to read a file in a Notification Service Extension, currently there is no way to handle that.

Feature Description

You can easily add this as an option or another method to write into or read from the shared app group directory like this (in Swift)

FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: "group.my.app.onesignal")

It will return the URL that the plugin can facilitate it like any other URL it's handling.