Can access file methods aka refactor into a proper 'File Serving API'

This issue is to implement a new method to allow a check to be made if a user can access a file. The method is:

• \file_storeage\can_access_file();` e.g. `$fs->can_access_file($file)`

The use case here is there are situations where we want to be able to check if a file can be accessed by a user in a particular context without sending the file. Currently there is no way to do this,

A specific use case is if a filter wants to check that a user can access a particular file embeded in some content and transmute that file prior to delivery OR deliver another file in it's place (possibly from outside of Moodle).