[cross_file] Add a base class feature to provide a file path  #196306

Currently, there is no way to know if the platform implementation for XFile.uri can be used as a file path. A file system implementation can provide a file path with FileSystemXFile.path, but a scoped storage implementation doesn't have an equivalent. For example, the URI of cross_file_darwin's security scoped implementation (a scoped storage implementation) can be used as a file path.

This issue proposes adding a new base class method that allows a platform implementation to provide a file path if possible, regardless of it being a file system or scoped storage implementation. Something along the lines of:

class PlatformXEntity {
  // ...
  String? get asFilePath => null;
}

class XEntity {
  // ...
  String? get asFilePath => platform.asFilePath;
}

comment for context: flutter/packages#13147 (comment)