Allow reading from/writing to iOS shared app group directory

# 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)

```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.

No files or tests are named. Start by locating the plugin's iOS file-access implementation and reviewing how existing URL-based reads and writes are exposed; then assess how the FileManager application-group container should be surfaced. Done means callers can read from and write to the shared app group directory through the plugin, with appropriate iOS coverage.

Written by the indexing model from the issue text.