Create a Functional test for replacing Drupal Core's "assets://" stream wrapper with a Flysystem implementation.

This is postponed on core supporting this in 🐛 AssetRoutes::routes() assumes stream wrapper implements ::getDirectoryPath() Active .

Adding a couple of open issues that are also tangentially related to this one.

@bradjones1 Just so I'm clear, is the intent for this MR to go along with the MR in 🐛 AssetRoutes::routes() assumes stream wrapper implements ::getDirectoryPath() Active ?

Yes - #4