vfs: Introduce PinState db storage #6815

vfs: Introduce PinState db storage #6815

The idea is to allow folders (and later maybe files?) to be
- pinned to be available locally
- pinned to be online only
- inherit their pin from the parent

Where this pinning only controls the default for new files.
Subfolders may have a different pin state, and contained files
may be hydrated or dehydrated based on user actions.

This value is stored in a new 'flags' table. The idea is to store
data there that doesn't necessarily exist for each metadata entry.
The selective sync state could be migrated to this table.