Please add a PHPDoc that explains what this means: a storage that have dynamic options. Hmm, it still sounds a bit strange. I wonder if we should rather add a special "remoteUser" and "remotePassword" settings in the config arrays that are auto-populated when they exist. Then it would just be a matter of detecting whether these settings exist instead of calling `isDynamic`, especially that this is specific to external storage.

@icewind1991 what do you think ?