`WiFiStorage`: need API to remove a directory

[vshymanskyy]

When working on the multi-platform Preferences library, I found that there is no way to clean up a directory using WiFiStorage.
Furthermore, there's no API to enumerate files (globally, or in a specific directory), so I cannot implement the cleanup logic.
This, along with #239 makes the WiFiNINA backend for Preferences incomplete. It would be great to fill the gap.