Cross-platform, high level file I/O module

Is your feature request related to a problem? Please describe.
There is the problem that in Android 11 the standard file I/O will not be supported anymore on external storage.
All file access on external storage must be done with the SAF (Storage Access Framework)
https://developer.android.com/about/versions/11/privacy/storage

Describe the solution you'd like
I would like to have an abstract, cross-platform, high level module for handling file I/O.
Files should not be referenced with a file path but rather with an URI.
On Android, they would be content URIs (content://), on other platforms, we could use file URIs (file://) or whatever fits the platform.

The module should provide about following functions:

• read a text file (with handling encoding and EOL conversion)
• read a binary file
• write a text file (with handling encoding and EOL conversion)
• write a binary file
• create a file
• delete a file
• create a folder
• get content of a folder
• delete a folder
• check for existence
• info about a file (size, timestamps, flags, etc.)

Start with the Android 11 Storage Access Framework documentation linked in the issue, then survey Toga's existing platform abstractions for a suitable integration point. Done means providing the requested cross-platform URI-based file and folder operations, including text encoding, EOL conversion, metadata, and Android external-storage support.