[feature] Storage Access Framework

[opk12]

What about supporting SAF?

• To easily open the SD card. It is a bit inconvenient to enable the root directory in the settings and navigate to /storage/ABCD-1234/.
• To access a network filesystem via a Document Provider app. For example, I use WebDAV and would install DAVx5; many other protocols are supported by RoundSync (including SFTP, which was asked for in [Feature request] SFTP #143).