Support storing graph via arbitrary location / sync provider on iOS

iOS provides “file bookmarks”, which grant apps persistent read/write access to files and folders managed by other apps. Letting users store their logseq graph by choosing a folder would let logseq users on iOS choose to store their graph in Google Drive, Dropbox, or anywhere else they choose.

More about this here: Beyond the Tablet: Seven Years of iPad as My Main Computer - MacStories - Part 3