Allow for flysystem plugin definitions to be altered

Added the line in a new merge request

Can you provide the use case for this? Asking because of the number of failing tests on Drupal 10.1 and 10.2. Use cases would be helpful in determine if the tests are failing because of this patch, or for other reasons, as well as writing tests to cover this alteration.

I'm sorry for the changes; I undid them. I mistakenly thought that the merge request was missing commits (and rebasing would help to fix the tests).

The tests are not failing because of this patch; they are the same as found on https://www.drupal.org/node/2489154/qa → Outside the scope of this ticket, but we should fix the tests and move to GitLab CI ( https://www.drupal.org/about/core/blog/drupalci-and-all-patch-testing-wi... → )

About the changes provided in this ticket/merge request, it allows others to alter the (plugin) definition by implementing a hook (i.e., hook_flysystem_plugins_alter()). This is done in Drupal core in many classes that extend the \Drupal\Core\Plugin\DefaultPluginManager, see for instance: https://git.drupalcode.org/project/drupal/-/blob/11.x/core/lib/Drupal/Co...

@ Watergate → . I'm going to mark this issue as postponed for the moment, I'm in the (hopefully) final stages of refactoring version 3.0 of this module to:

* Utilize League\Flysystem version 3.0
* Drop the current plugin system (since plugins were eliminated in League\Flysystem 2.0) and implement a new plugin system for adapters for version 3.0 (I'm working on that now)
* Implement a Flysystem StreamWrapper that is compliant with Drupal\Core\StreamWrapper\StreamWrapperInterface
* Decorate Drupal\Core\File\Filesystem to allow a seamless switching between file management using Drupal Core and file management using Flysystem
* Allow for configuration of League\Flysystem adapters through the UI

This is a major overhaul that is long overdue.

I'm happy to notify you when the Adapter plugin system is in place if you'd like to modify your PR for version 3.0, as I have not built in any drupal hooks but am open to doing so.