Add SFTP support to core module

Patch attached for easier composer use

This has been decoupled from the base League\Flysystem library, and will be pulled out of version 3.0.x of the Drupal module.

It will be implemented as a separate project, which will have a dependency on the D11/D10 (depending on timing of progress of the base refactor of the module). The main refactor issue can be followed:

https://www.drupal.org/project/flysystem/issues/31958 →
https://www.drupal.org/project/flysystem/issues/3378738 🌱 Planning for Release 3.0.0, Drupal 10.1+ Compatibility, Flysystem v3.1 Compatibility Needs work

Please don't hesitate to reach out in the Drupal Slack channel for the Flysystem module, as I welcome the assistance in refactoring v2 of the module for Drupal 10+ support in v3+ of the module, which will upgrade implementation dependencies from version 1.x to version 3.x, and would love some assistance in getting there.