leverage flysystem for all filesystems

[dbu]

for version 3, we should drop our custom AWS integration and any other storage in favor of leveraging flysystem. this will allow to remove a lot of code that flysystem is doing better than us.

we still want to define our own interface to define the interactions with storage to not allow non-flysystem storage.

this would also close #1457

see how to leverage https://flysystem.thephpleague.com/docs/usage/public-urls/ to handle image caches stores in flysystem.